Genesis 15:16

And the fourth generation: After they will have been exiled to Egypt, they will be there for three generations, and the fourth will return to this land (Mishnath Rabbi Eliezer ch. 5), for in the land of Canaan He spoke with him, and formed this covenant, as it is written (above verse 7): “to give you this land to inherit it.” And so it was: Jacob descended to Egypt. Go forth and figure his generations: Judah, Perez, and Hezron, and Caleb the son of Hezron was one of those who entered the land (Sotah 11b).

for the iniquity of the Amorites will not be complete: that they should be sent out of their land until that time, for the Holy One, blessed be He, does not punish a nation until its measure is full, as it is said: (Isa. 27:8):“When her measure is full, when You send her away, then You will strive with her.” - [from Zohar, vol. 1, p. 113b]

And in the fourth generation of thy sons they will return hither to inherit; because the guilt of the Amoraah is not yet complete.

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.

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.

כי לא שלם עון האמורי עד הנה, the time when the Emorite is due for punishment will not be until then. G'd is very patient with sinners, giving them a great deal of time to mend their ways. The punishment for sin is called עון, as we know already from Kayin (Genesis 4,13) who said of the punishment for his having murdered Hevel, his brother, גדול עוני מנשוא, "my punishment is too great for me to endure." We also find that the word חטאת is used both as sin and as atonement for sin, just as the word עון sometimes means "sin," and sometimes "punishment." Compare Zecharyah 14,19 זאת תהיה חטאת מצרים. The reason the Torah singled out the tribe of the Emorite among all the Canaanite tribes is because it was the strongest of those seven tribes. The prophet Amos 2,9 also quotes G'd of having destroyed the Emorite on account of the Jewish people, although the same fate overtook the other Canaanite tribes also.