Looking back to 3:30-31, he says THE LAW and THE GRACE are one in God because God is one. They are not at odds. They work together.
4:1 - “What then” - basically he is about to prove the previous statement by showing abraham as an example.
Although Abraham wasnt given the mosaic law, he still was given the promise of the covenant which is the beginnings and foundation of the law (circumcision).
