אחר הדברים האלה, the events related in the previous paragraph when Avram killed the 4 kings. G’d said to him: “do not be afraid, Avram, etc.”

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.