The distinction between the promise to Abraham and the Law through Moses explained in the previous passage does not mean they are opposed to each other (v21). What the Judaizers have done, essentially, is make the Law more important than the promise to Abraham. In this light, Paul writes the famous words in verses 26-29, celebrating the unity that the Church should be creating. The Judaizers, on the other hand, are creating division in the Church!
In the beginning of chapter 4, then, Paul speaks of receiving the promise to Abraham in terms of an inheritance. The Law was like a guardian or pedagogue, looking after God’s people while they grew up, but in Christ they become heirs with him of the promise. This movement from bondage to freedom is a new exodus for God’s people!
In summary, the Law was a slaver (3:22), a guardian (3:24-25), and a teacher (4:2). But now God has given us the Holy Spirit so we can truly call God our father just like Jesus could (4:6). In that prior tutelage, we were enslaved by the world, but God bought us back (redeemed us).