Ramban, on Gen 15:16

“But in the fourth generation they shall return hither: for as yet the iniquities of the Amorrhites are not at the full until this present time.”

“AND IN THE FOURTH GENERATION THEY SHALL COME BACK HITHER. After they are exiled into Egypt, they will be there for three generations. And thus it happened; Jacob was exiled into Egypt. Go and reckon his generations: Judah, Peretz, Chetzron, and Caleb the son of Chetzron was amongst those who entered the Land. Thus the language of Rashi. But this is not correct at all. The correct interpretation appears to be that the fourth generation refers to the Amorite whose sin will then become full, for from the day of the decree He prolonged the time of the Amorite, as He visits iniquity upon the third and fourth generation. Had the Amorites repented of their iniquities He would not have utterly destroyed them. Rather, they would have been a levy of bondservants, or they might have gone elsewhere. THE INIQUITY OF THE AMORITE. He mentioned the strongest among them, Whose height was like the height of the cedars. The Israelites would not be able to overpower him until his measure of sin was full and his own iniquities will ensnare him. Moreover the Amorite was the first one to be captured by them, and it was his land which they inherited first.”

Imported from an open dataset — not yet checked against the printed edition.