Galatians 4:6

because ye are sons--The gift of the Spirit of prayer is the consequence of our adoption. The Gentile Galatians might think, as the Jews were under the law before their adoption, that so they, too, must first be under the law. Paul, by anticipation, meets this objection by saying, YE ARE sons, therefore ye need not be as children (Gal 4:1) under the tutorship of the law, as being already in the free state of "sons" of God by faith in Christ (Gal 3:26), no longer in your nonage (as "children," Gal 4:1). The Spirit of God's only Begotten Son in your hearts, sent from, and leading you to cry to, the Father, attests your sonship by adoption: for the Spirit is the "earnest of your inheritance" (Rom 8:15-16; Eph 1:13). "It is because ye are sons that God sent forth" (the Greek requires this translation, not "hath sent forth") into OUR (so the oldest manuscripts read for "your," in English Version) hearts the Spirit of His son, crying, "Abba, Father" (Joh 1:12). As in Gal 4:5 he changed from "them," the third person, to "we," the first person, so here he changes from "ye," the second person, to "our," the first person: this he does to identify their case as Gentiles, with his own and that of his believing fellow countrymen, as Jews.

— Jamieson-Fausset-Brown Commentary (public domain)