Jamieson, Fausset and Brown on Galatians 4:31

31. So then--The oldest manuscripts read, "Wherefore." This is the conclusion inferred from what precedes. In Ga 3:29 and Ga 4:7, it was established that we, New Testament believers, are "heirs." If, then, we are heirs, "we are not children of the bond woman (whose son, according to Scripture, was 'not to be heir,' Ga 4:30), but of the free woman (whose son was, according to Scripture, to be heir). For we are not "cast out" as Ishmael, but accepted as sons and heirs.