New Defender's Study Bible Notes
15:16 fourth generation. It is not absolutely certain whether the 430-year term mentioned in the Abrahamic covenant begins with the coming of Abraham into Canaan or the descent of the children of Israel into Egypt. The former seems indicated by Galatians 3:16-17 in which case the “sojourning” mentioned in Exodus 12:40 and the “affliction” of Genesis 15:13 would apply to their total experience in both Canaan and Egypt. If this is the case, then the actual sojourn in Egypt would be only 215 years (from the time of Abram’s entry into Canaan to the birth of Isaac was 25 years; Isaac was 60 years old at Jacob’s birth; and Jacob was 130 years old when he and his children migrated to Egypt, a total of 215 years–see Genesis 12:4; 21:5; 25:26; 47:9). The “fourth generation” consisted of men whose great-grandfathers had been among the seventy Israelites who entered Egypt. Even if the children of Israel actually stayed 400 years in Egypt, the life span in those days was still around 100 or more (Moses died at 120), so it would have easily been possible for people of the fourth generation still to be living at the time of the exodus.
There is also the problem of whether 215 years could have been enough time for the Israelite population to grow from about seventy to about two million. Although this seems unlikely, it would have been possible if the average family size had been about eight children (remember that Jacob himself had twelve) and the average life span still about one hundred years, with parents living to see their great-grandchildren grow to maturity. In contrast, consider the notes at Exodus 6:17 and 12:41.