Therefore if: Ei tis oun: 

The “therefore” (oun) is probably connected with the exhortations to unity in Php 1:27. Paul now proceeds to enumerate the resources they can tap into to maintain and maximize unity in their local body at Philippi and which can fulfill his command in  Phil 2:2 to "make my joy complete".

Edwards sums up this section writing that...

Paul begins by appealing to they relationship in Christ. All these four terms for affection appear to be fairly similar and all four (really five) are distinctly peculiar to believers. Based on the encouragement which is in Christ, the comfort which comes from divine love, the oneness (fellowship) of an those drinking from the same Spirit, and the tender mercies of these believers, they are to live a certain way. They are to live in love and harmony, not because of their natural fondness for one another (though that may be there), but because of they divine responsibilities as members of the household of God. This unity is not the result of natural oneness but supernatural bonding. This is why Paul's appeal for unity begins by focusing upon their relationship in Christ, not their relationship towards one another. (Philippians)

Guzik adds that the

"Therefore draws back to what Paul has built on in Php 1:27-30, telling the Philippians how to stand strong for the Lord against external conflicts.  Now he tells them how to act against internal conflicts in the body of Christ."

Vincent says that the "therefore" is there for it is both the saint's

"duty and privilege (to) fulfill my joy, and show yourselves to be true citizens of God's kingdom by your humility and unity of spirit."

"If” in each of the 4 uses in this verse is the same Greek word "ei" which is what is referred to as a first class conditional particle which means that what follows equates with a fulfilled condition. It follows that the first class conditional particle can usually be accurately translated with “since”, "so then", “in view of the fact” or “If such-and-such is true―and I know that it is …”

All four characteristics in this verse are indisputable facts - certainties not "maybes" and are reminders of the resources God has provided for us. There was not a hint of doubt in Paul's mind as he penned these thoughts. In these four succinct clauses Paul sets forth a powerful motive for harmony in the Christian community. Because the saints at Philippi are each recipients of these 4 benefits (foundation stones), they have the resources as well as the responsibility to carry out what Paul is about to command in the next verse, the end result being unity which makes Paul's joy full. Parenthetically this order (provision provided before practice commanded) illustrates how God never asks us to do what He does not also enable us to do. God is good and wise. In sum, because these four things are true the Philippian believers, they can be "of the same mind".

Why is Christian unity so important? In John 17, Jesus prayed for unity four times for His disciples clearly emphasizing the importance our Lord Jesus placed on unity. Jesus asked His Father to

"keep them in Thy name, the name which Thou hast given Me, that they may be one, even as We are" (Jn 17:11) and that His disciples "may all be one; even as Thou, Father, art in Me, and I in Thee, that they also may be in Us; that the world may believe that Thou didst send Me. And the glory which Thou hast given Me I have given to them; that they may be one, just as We are one, I in them, and Thou in Me, that they may be perfected in unity, that the world may know that Thou didst send Me, and didst love them, even as Thou didst love Me." (Jn 17:21 22 23)

Why did Jesus hold Christian unity in such high esteem? Read the verses again.

Matthew Henry sums up this section stating that Paul

"presses them largely to like-mindedness and lowly-mindedness, in conformity to the example of the Lord Jesus, the great pattern of humility and love."

if there is any encouragement in Christ: paraklesis en Christo: (3:3; Lu 2:10,11,25; Jn 14:18,27; 15:11; 16:22-24; 17:13; Ro 5:1,2; Ro 15:12,13;  2Co 1:5,6; 2:14; 2Th 2:16,17; Heb 6:18; 1Pe 1:6-8)

The first foundation stone for unity is "encouragement in Christ"

"So if in Christ there is anything that will move you," (NJB),
"Is there any encouragement from belonging to Christ?" (NLT),
"Now if your experience of Christ's encouragement and love means anything to you" (Phillips),
"In view of the fact that there is a certain ground of appeal in Christ which exhorts" (Wuest),
"SO BY whatever [appeal to you there is in our mutual dwelling in Christ" (Amp),
"If the fact that you are in Christ has any power to influence you" (Barclay)

Spurgeon introduces this section writing that...

the Holy Spirit, during the present dispensation, is revealed to us as the Comforter. It is the Spirit's business to console and cheer the hearts of God's people. He does convince of sin; he does illuminate and instruct; but still the main part of his business lies in making glad the hearts of the renewed, in confirming the weak, and lifting up all those that be bowed down. Whatever the Holy Ghost may not be, he is evermore the Comforter to the Church; and this age is peculiarly the dispensation of the Holy Spirit, in which Christ cheers us not by his personal presence, as he shall do by-and-bye, but by te indwelling and constant abiding of the Holy Ghost the Comforter. Now, mark you, as the Holy Spirit is the Comforter, Christ is the comfort. The Holy Spirit consoles, but Christ is the consolation. If I may use the figure, the Holy Spirit is the Physician, but Christ is the medicine. He heals the wound, but it is by applying the holy ointment of Christ's name and grace. He takes not of his own things, but of the things of Christ. We are not consoled to-day by new revelations, but by the old revelation explained, enforced, and lit up with new splendour by the presence and power of the Holy Ghost the Comforter. If we give to the Holy Spirit the greek name of Paraclete, as we sometimes do, then our heart confers on our blessed Lord Jesus the title of the Paraclesis. If the one be the Comforter, the other is the comfort. (Spurgeon's Sermon on "Consolation in Christ")

"Lord, Thy death and passion give
Strength and comfort in my need,
Every hour while here I live,
On Thy love my soul shall feed."

Encouragement (3874) (paraklesis from parakaléo = beseech <> pará = side of + kaléo = call) refers to calling to one's side and means solace, comfort, consolation, exhortation. Webster says that encouragement is the act of inspiring with courage or giving one support, confidence or hope.

Marvin Vincent has a detailed note on this word group writing that parakaleo literally means...

"a calling to one’s side to help; and therefore entreaty, passing on into the sense of exhortation, and thence into that of consolatory exhortation; and so coming round to mean that which one is summoned to give to a suppliant—consolation. Thus it embodies the call for help, and the response to the call. Its use corresponds with that of the kindred verb , to exhort or console...In some instances, the meaning wavers between console and exhort. In the sense of exhortation or counsel, the noun may be found in Acts13:15; Ro 12:8; Heb 13:22. The verb, in Acts 2:40; 11:23; 14:22; Ro 12:8; Titus 2:15. Neither the noun nor the verb appear in the writings of John, but the kindred word the Paraclete, Comforter, or Advocate, is peculiar to him. It should be noted, however, that the word comfort goes deeper than its popular conception of soothing. It is from the later Latin , to make strong. Thus Wycliffe renders Lu 1:80, “the child waxed, and was comforted in spirit”  and Tyndale, Lu 22:43, “there appeared an angel from heaven comforting him” (AV., strengthening).

The comfort which Christ gives is not always soothing. The Holy Spirit, the Comforter, is to convince of sin and of judgment. Underlying the word is the sense of a wise counsel or admonition which rouses and braces the moral nature and encourages and strengthens it to do and to endure. When, therefore, Christ says “they that mourn shall be comforted,” he speaks in recognition of the fact that all sorrow is the outcome of sin, and that true comfort is given, not only in pardon for the past, but in strength to fight and resist and overcome sin. The atmosphere of the word, in short, is not the atmosphere of the sick-chamber, but the tonic breath of the open world, of moral struggle and victory; the atmosphere for him that climbs and toils and fights.

Robertson writes that

"If one's own life in Christ does not stimulate the soul to the noblest effort, it is useless to go on with the appeal."

Two of the paraphrases pick up this same thought --

"If the fact that you are in Christ has any power to influence you" (Barclay)

"If then your experiences in Christ appeal to you with any force.." (Lightfoot)

Is there encouragement in Christ? Of course there is. Godly Simeon called  Jesus the "the Consolation (paraklesis) of Israel" (Lu 2:25)

Paul taught that

"just as the sufferings of Christ are ours in abundance, so also our COMFORT (paraklesis) is abundant through Christ." (2Co 1:5)

Every believer has received encouragement, exhortation, and comfort from Christ Who is like an artesian well that effortlessly, endlessly flows through us as we surrender our will to His sweet will and in the context as the saints at Philippi enter into the reality of Christ Who is now their life, and the result of this common experience will serve to draw them together and unite them.

F B Meyer writes regarding the bonds of unity in a local body that...

The first bond is the consolation which is in Christ. For consolation let us substitute exhortation, or, better still, persuasiveness, so that we might put it that the first bond of Christian fellowship is Christ's persuasiveness. That Jesus Christ is interested in every Church fellowship is obvious, but we do not always realise how much He is always doing to persuade us to main-rain it. Have there not been times in your life when you have been greatly incensed, but have realised that there was a voice speaking within your heart, and a gentle influence stealing over you, a yearning towards the brother about whom you had cherished hard and unkind feelings? That has been the persuasiveness of Christ. It is He who has besought you to check that word, to refrain from writing that letter, to abandon that bitter and offensive way which had seemed so befitting a method of repaying your enemy to his face. It was Christ who was persuading you to drop the weapon from your hand, and to reach it out in brotherhood, and this because He was so eager to keep the unity of the Spirit unbroken in the bond of peace. (The Epistle to the Philippians )

if there is any consolation of love: ei ti paramuthion agapes: (Ps 133:1; Jn 15:10-12; Acts 2:46; 4:32; Gal 5:22; Eph 4:30-32; Col 2:2; 1Jn 4:7,8,4:12, 4:16)

 The second foundation stone for unity is consolation of love (agape) where agape is God's love (cf the quality & quantity of that love in Jn 3:16 Torrey's Topic "Love of God")

"since there is a certain tender persuasion that comes from divine love" (Wuest),
"by whatever persuasive incentive there is in love" (Amp),
"if there is any persuasive power in love" (Weymouth),
"If love has any power by its tenderness to stir your hearts, then listen to me." (Robertson)

Pentecost phrases it this way...

The fact that Christ loved me ought to move me to love the brethren. We could paraphrase the phrase, “if any comfort of love,” with these words, “if the love of Christ exerts any persuasive power, if love supplies an incentive or gives encouragement,” then fulfill my joy by loving one another. The fact that God loved me, as unlovely as I was, ought to move me to love the brethren.  (Pentecost, J. D. The Joy of Living: A study of Philippians. Kregel Publications)

Consolation (3890) (paramuthion from para = towards + muthéomai = to speak, which is from múthos = a tale, myth, speech) literally describes speaking closely to someone. The idea is to speak to someone coming close to their side. The basic sense speaking to someone in a friendly way. It refers to that which causes or constitutes the basis for consolation and encouragement. 

Paramuthion "indicates a greater degree of tenderness than" the preceding word "encouragement" (paraklesis).

One Greek lexicon defines paramuthion as an assuagement ( = lessening the intensity of something that pains or distresses).

Friberg defines paramuthion...

 as persuasive power that points to a basis for hope and provides incentive. (Analytical Lexicon of the Greek New Testament)

Consolation is the attribute of agape love of God that alleviates grief, the sense of loss, trouble, etc.

Vine says that "consolation" is

"that tender cheer, imparted as the effect of “love” (agape practical love)."  (Vine, W. Collected writings of W. E. Vine. Nashville: Thomas Nelson )

Rienecker adds intriguing note that the preposition (para = beside) may have the force of aside and pictures consolation that draws one's mind "aside" from their cares and concerns.

MacArthur adds that paramuthion

"portrays the Lord coming close and whispering words of gentle cheer or tender counsel in a believer’s ear."

Wuest has an excellent note

"We have here the subjective genitive construction, in which the noun in the genitive case, “love,” produces the action in the noun of action, “consolation.” That is,

the tender persuasion and encouragement which exhorts to unity among the Philippians, comes from God’s love for them.

Their realization of divine love which reached down and saved them, should urge them to live in a spirit of unity with one another. In addition to that, this divine love produced in the hearts of the Philippian saints by the Holy Spirit (cf Ro 5:5, Gal 5:16), should cause them to so love each other with a love that impels one to sacrifice one’s self for the one loved, that their little differences will be ironed out, and they will live in unity with one another.” (Wuest, K. S. Wuest's Word Studies from the Greek New Testament: Eerdmans)

F B Meyer writes regarding the bonds of unity in a local body that...

The second bond is the comfort of love. The Greek word will bear this rendering--if you know the tender cheer that love gives; that is, see to it that you maintain the bond of Christian fellowship by meeting your fellow Christians with the tender cheer of love. We all know what tender cheer is, when men have been out all day and tried, almost beyond endurance. As they come out of the storm, the depression of their spirit and their health may have conspired to reduce them to the lowest depth of darkness--then as the door opens, and they see the ruddy glow of the fire, and the wife comes to meet them, and the child is there with its prattle, for a moment it seems almost worth while having known the weariness and depression because of the contrasted cheer that greets them. All around us in the world are Christian hearts which are losing faith; many hands hang down, and knees shake together. Let us see to it that by the kindly cheer of a smile, the grasp of a hand, the welcome of a word, we do something to draw those people into the inner circle of Christian love. (The Epistle to the Philippians )

if there is any fellowship of the Spirit: ei tis koinonia pneumatos: (Ro 5:5; 8:9-16;8:26; 1Co 3:16; 6:19,20; 12:13; 2Co 13:14; Gal 4:6; Eph 1:13,14; 2:18-22; 4:4; 1Pet 1:2;1Pet 1:22,23; 1Jn 3:24)

 The third foundation stone for unity is "fellowship of the Spirit"

"in view of the fact that there is a certain joint-participation with the Spirit in a common interest and activity" (Wuest),
"if your fellowship in the Spirit is a living reality." (Lightfoot) 
"if you really are sharing in the Holy Spirit" (Barclay)

Paul says, in effect, “If there is any such thing as communion with the indwelling Spirit, or if your consciousness of fellowship with the Holy Spirit who dwells within is a reality in your life, and it most certainly is, then fulfill my joy by your love for one another.”

Fellowship  (2842) (koinonia from koinos = common, shared by all) (Click for an in depth word study of koinonia) means a close association involving mutual interests and sharing (communion, fellowship, partnership). Koinonia is an intimate partnership, a common eternal life or joint participation with common interests and mutual, active participation. This dynamic is effected by Holy Spirit’s working in and through individual saints in the body to produce unity (1Co 3:16, 12:13, 2Co 13:14,cf 1Jn 1:4-6)

One translation has

"If communion with the Spirit of love is not a mere idle name, but a real thing" 

Robertson says that

"If we have any partnership in the life and blessings of the Holy Spirit, then we are ready to listen to Paul's plea for unity."

Paul is reminding them that the fellowship of the Holy Spirit is a blessed reality, not merely a beautiful idea. Remember every genuine believer at Philippi (and in the body of Christ today) has received the Holy Spirit for as Paul writes in his epistle to the saints at Rome...

the love of God has been poured out within our hearts through the Holy Spirit Who was given to us. (see note Ro 5:5)  

However, you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. But if anyone does not have the Spirit of Christ, he does not belong to Him.  And if Christ is in you, though the body is dead because of sin, yet the spirit is alive because of righteousness. 11 But if the Spirit of Him who raised Jesus from the dead dwells in you, He who raised Christ Jesus from the dead will also give life to your mortal bodies through His Spirit who indwells you. 12 So then, brethren, we are under obligation, not to the flesh, to live according to the flesh-- 13 for if you are