Here Apostle Sha'ul continues his premise from the previous chapter.
He proves that the Gentiles can be saved because Avraham already had the spiritual sign of the inward circumcision BEFORE HE WAS PHYSICALLY CIRCUMCISED.
Now, because Avraham was imputed righteous due to his faith, and not due to his obediance to Torah, did this then abrograte his responsbility to eventually comply to the letter of Torah? NO!
Avraham, who was born of faith, and not by works, then PROVED HIS FAITH BY BECOMING PHYSICALLY CIRCUMCISED.
So Romans 4, shows two awesome truths: We are justified by faith but obey the letter of Torah as a result of faith even as our father Avraham did
Both Jew and Gentile can be saved, even as it was in Exodus 12, because Avraham was justified by faith long before he was physically circumcised.