4:1 What I am saying is that as long as the heir is a child, he is no different from a slave [under the law; child (heir) like a person without rights], although he owns the whole estate.
4:2 He is subject to guardians [official who cared for minor] and trustees [person who handled property/estate of child] until the time set by his father [age the father determined son could receive inheritance].
4:3 So also [in the same way], when we were children [before trusting Christ], we were in slavery under the basic principles of the world [phrase probably alludes to the Jewish law with its emphasis on restrictions and rituals].
4:4 But when the time [Gr. chronos: minutes and hours] had fully come [God timed the moment of Christ’s birth when all conditions were right; a fixed point in time predetermined by God], God sent [on a mission] His [emphasizes Christ’s deity] Son, born of a woman [Jesus was born of the virgin Mary; emphasizes His humanity; Jesus entered the world clothed in humanity], born under law [reminds us that Christ was born as a Jew under the Jewish law, yet perfectly obeyed the law],
Note: Why did God chose the particular time He did to send His Son?
• Roman Empire enjoying the “Roman Peace” which was providing political stability.
• Practically everyone in known world spoke Greek, making it easier to communicate the gospel.
• People could travel in safety throughout the Roman Empire over a network of good roads because the world was a peace.
• People were looking for answers to life’s bigger questions and were receptive to the message of salvation.
• In addition, the Jews were anticipating the arrival of the Messiah.
4:5  to redeem [purpose of Christ’s coming; liberation at a price] those under law [law of Moses (Jews) and natural law that was written on their hearts (Gentiles: Rom. 2:14-15)],  that we might receive [as those adopted into God’s family] the full rights of sons [adoption; with all the privileges and rights of family members; from slavery to sonship].
4:6 Because [“as a result/proof of your being sons”] you are sons, God sent the Spirit [presence of the Holy Spirit is evidence we have been adopted and received the rights of sons (v. 5)] of his Son into our hearts [center of our physical, mental, and spiritual lives], the Spirit who calls out, “Abba [Aramaic: “daddy” (a word associated not so much with infancy as with intimacy) — describes close relationship with God], Father.”
4:7 So [because of Christ] you are no longer a slave, but [signals a change of status] a son [in God’s family]; and since you are a son, God has made you also an heir [with full rights and privileges].
4:8 Formerly [Paul reminded the Galatians of their pagan past by using two phrases], when  you did not know God,  you were slaves to those who by nature are not gods [the Galatians had been so involved in idolatry that when Paul and Barnabas first visited the area, the Galatians thought that Paul was the god Hermes and that Barnabas was Zeus (see Acts 14:11-13)].
4:9 But now [God had delivered the Galatians out of spiritual bondage to false gods] that you know God — or rather are known [approved or acknowledged by God; a reminder that God had taken the initiative to know them] by God — how is it [Paul wanted to know how they could turn from freedom in Christ to their former way of life] that you are turning back to those weak [things that could not provide righteousness] and miserable [gave the Galatians no spiritual riches or privileges before God] principles [cf. Col. 2:8]? Do you wish to be enslaved by them all over again?
Note: It made no sense to Paul that after God had delivered the Galatians from their bondage to false gods, they wanted to put themselves under bondage to the requirements of the Mosaic law. They were merely trading one kind of spiritual slavery for another.
4:10 You are [the Galatians were already reverting to the Mosaic law by…] observing special days [reference to Sabbath regulations] and months [reference to new moon rituals] and seasons [probably a reference to annual celebrations like Passover, Pentecost, and Tabernacles] and years [probably a reference to the jubilee and sabbatical years commanded by the law]!
Note: The Galatians’ observance of these Jewish rituals was evidence that they were sliding back into spiritual slavery. Paul’s purpose in mentioning these terms was to show that religious observances added to faith in Christ gain no advantage toward salvation.
Note: One reason why people are attracted to legalism and works is because it enables them to do things and measure external results. Legalism gives people an opportunity to look good and can easily lead to pride.
4:11 I fear [Paul feared that if the Galatians returned again to the spiritual slavery of the Mosaic law then his work among them had been in vain; their return to legalism was tantamount to a rejection of the gospel they had professed] for you, that somehow I have wasted my efforts on you.
4:12 I plead [shows earnestness of Paul’s appeal] with you, brothers [an affectionate term], become like me [free of Jewish legal observances], for I became like you [in order to win them to Christ (cf. 1 Cor. 9:19-22)]. You have done me no wrong [see following verses re: how they had received Paul in his physical illness].
4:13 [cf. Acts 13:14–4:23] As you know, it was because of an illness [we do not know the nature of Paul’s illness] that I first preached the gospel to you.
4:14 Even though my illness [suggestions include malaria, epilepsy, eye disorder] was a trial [Paul’s illness had been an imposition or burden to those who cared for him] to you, you did not treat me with contempt or scorn. Instead, you welcomed me as if I were an angel of God, as if I were Christ Jesus himself.
4:15 What has happened to all your joy [there is no joy in legalism]? I can testify that [note the affection the Galatians had for Paul…], if you could have done so, you would have torn out your eyes [some conjecture that Paul’s illness was some type of eye disorder] and given them to me.
4:16 [Galatians so deceived by Judaizers that they saw Paul in a different light…] Have I now become your enemy [a persona non grata] by telling you the truth?
4:17 Those people [the Judaizers] are zealous to win you over [like a boy courting a girl], but for no good [Judaizers had wrong motives]. What they want is to alienate [literally “to shut out”] you from us [Paul and his companions], so that you may be zealous for them [not for Christ].
4:18 It is fine to be zealous, provided the purpose is good, and to be so always and not just when I am with you.
4:19 My dear children [a term of endearment; Paul had brought them to faith in Christ by preaching the gospel to them (cf. 1 Cor. 4:15)], for whom I am again in the pains of childbirth until Christ is formed in you [refers to spiritual maturity],
4:20 how I wish I could be with you [Paul preferred a face-to-face encounter] now and change my tone, because I am perplexed about you [Paul was at his wit’s end and frustrated by the Galatians’ defection from the gospel of grace]!