In Romans 4, Paul is still wrestling with those who want to make righteousness with God a matter of works (i.e. circumcision) rather than a matter of faith. To buttress his argument, Paul goes all the way back to the patriarch Abraham to explore just what it was about Abraham that justified him.
What Paul emphasizes is the scripture, "Abraham believed God, and it was credited to him as righteousness." Abraham was not justified by works, but by faith. He was reckoned righteous through his faith not on the basis of his circumcision - for he was still uncircumcised when he was declared righteous by God.
If Abraham was considered justified while uncircumcised, that can give hope to uncircumcised Gentiles as well! In fact, Paul goes on to argue that Abraham "is the father of all who believe but have not been circumcised." (vs. 11) What really makes someone a descendant of Abraham isn't the state of his circumcision, but the state of his heart! Paul is saying that if you have faith in Christ, YOU are Abraham's descendant!
The good news is that - whether we are Jewish or Gentile - Abraham "is the father of us all." (vs. 16)