Commentaire sur La Genèse 15:16

ודור רביעי BUT A FOURTH GENERATION — i.e. after they go into exile in Egypt they will be there three generations, and the fourth will return to this land (הֵנָה hither). For He was then speaking to him in the land of Canaan and it was there that He made this covenant, as it is written (v.7) “to give thee this land to inherit it”. Thus it really was: Jacob went down to Egypt. Go and count up his generations: Judah, Perez, Hezron— and Caleb (whose father Jephuneh is identified with Hezron, see Sotah 11b) was amongst those who entered the land of Canaan.

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.³²⁶Since Chetzron was among those who went down to Egypt (further, 46:12), they were in Egypt for only two generations. And if “the dwelling in a strange land” is to be reckoned as beginning with Abraham, there are seven generations from Abraham to Caleb: Abraham, Isaac, Jacob, Judah, Peretz, Chetzron and Caleb. (Tur.)
The correct interpretation appears to be that the fourth generation refers to the Amorite whose sin will then become full,³²⁷Thus enabling Israel to return and take the land from him. (Tur.) 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.

ודור רביעי ישובו הנה, anyone interpreting these words as referring to the fourth generation of Israelites is in error. Seeing that G'd had already told Avram that the process of inheriting the land of Canaan would not be complete until after 400 years from the time his true seed had been born, what was the point in telling him how many generations down the line this would occur? Until the 400 years would be up, G'd's promise would not be due for fulfillment in any event! The reason G'd mentioned the "fourth generation" was to explain to Avram why the delay in fulfilling His promise would require 400 ears. A generation in those days was considered a period of 100 years, as we know from Ediyot 12,9 so that 400 years equal 4 generations. We know from the second of the Ten Commandments, (Exodus 20,5 that G'd frequently extends His patience to sinners and their offspring until the fourth generation, waiting if finally someone will redeem the errant parents and grandparents by returning to the true lifestyle approved by G'd. This is why G'd described the sin of the Emorites at this time as לא שלם, "not complete," in the sense of not yet irreversible, עד הנה until such time.