Genesis 15:16 - Matthew Poole's English Annotations on the Holy Bible

In the fourth generation; in the end of the four hundred years mentioned Genesis 15:13, a generation being at that time reckoned at one hundred years, or thereabouts. Or, in the fourth generation numbered from their going into Egypt, or from their leaving Canaan; which may possibly be implied by these words, they shall come hither. So Caleb was the fourth from Judah, and Moses the fourth from Levi, and so doubtless many others. The iniquity of the Amorites, i.e. of the people inhabiting Canaan. And the Amorites, one of those people, Genesis 15:21, are here put for all the rest, as Genesis 48:22 1 Kings 21:26 2 Kings 21:2 Amos 2:10, either because they were the greatest and stoutest of all, Amos 2:9, or because Abram dwelt among that people, Genesis 13:18, Genesis 14:13. All men's sins are kept by God as in a book of remembrance, not one of them is lost; and as God exactly observes the number and measure of men's sins, so he determines within himself how far and how long he will bear with sinful men or nations, and what shall be the period of his patience; and when that comes, their measure is full, and their destruction infallibly comes. See Jeremiah 51:13 Matthew 23:32 1 Thessalonians 2:16.