But in the fourth generation they shall come hither again: for the {e} iniquity of the Amorites is not yet full.

(e) Though God tolerates the wicked for a time, yet his vengeance falls on them when the measure of their wickedness is full.

Wesley's Notes

15:16 They shall come hither again - Hither to the land of Canaan, wherein thou now art. The reason why they must not have the land of promise in possession till the fourth generation, is because the iniquity of the Amorites was not yet full. The righteous God has determined, that they shall not be cut off till they are arrived to such a pitch of wickedness; and therefore till it come to that, the seed of Abram must be kept out of possession.

Matthew Henry's Concise Commentary

15:12-16 A deep sleep fell upon Abram; with this sleep a horror of great darkness fell upon him: a sudden change. The children of light do not always walk in the light. Several things were then foretold. 1. The suffering state of Abram's seed for a long time. They shall be strangers. The heirs of heaven are strangers on earth. They shall be servants; but Canaanites serve under a curse, the Hebrews under a blessing. They shall be suffers. Those that are blessed and beloved of God, are often sorely afflicted by wicked men. 2. The judgment of the enemies of Abram's seed. Though God may allow persecutors and oppressors to trample upon his people a great while, he will certainly reckon with them at last. 3. That great event, the deliverance of Abram's seed out of Egypt, is here foretold. 4. Their happy settlement in Canaan. They shall come hither again. The measure of sin fills gradually. Some people's measure of sin fills slowly. The knowledge of future events would seldom add to our comfort. In the most favoured families, and most happy lives, there are so many afflictions, that it is merciful in God to conceal what will befall us and ours.