Rabbi Akiva used to say that a father, through his zechuyot, through his mitzvot (commandments), through his righteousness, could benefit his son with beauty, with strength, with money, with knowledge, and with years of life. A father who acts a certain way will leave a certain Beracha (blessing) for his children after him. There are times when Hashem rewards not by a specific number of years—that a certain Beracha will come after a certain number of years—but by a certain generation, when a Beracha is going to come to some of his descendants in the third or fourth generation. And a father, through his zechuyot, also could benefit his descendants a certain number of generations after him: his grandchildren or his great-grandchildren. And that means it's the time—it's the time for that reward. Hashem promises that a person is going to be rewarded his descendants in the future. That time could arrive by generation as opposed to years.
Like the pasuk (verse) of Yeshayahu says, “Hashem who called out the generations from the beginning,” which means Hashem arranges how things are going to work out according to the generations, from the dorot (generations), from the beginning. Even though it says that Hashem, in the Brit Bein HaBetarim, promised Avraham that eventually, after B'nei Yisrael go out and are enslaved in Egypt, they are going to be redeemed after 400 years, it also says that the fourth generation is going to leave there and return here to Eretz Yisrael. So it's talking about a particular generation that's going to leave, even though those two teachings seem to be at odds. 400 years is different from four generations. It doesn't automatically mean the same.
What we say is that the number of generations was the main focus, and that's what actually happened. And the number of years is really the secondary objective of the pasuk: the Galut (exile) we know in Yisrael did not last for 400 years; it only lasted 210 years. And the time of the 400 years is really from the birth of Yitzchak Avinu. So we see from here that the main calculation is that the fourth generation entered Eretz Yisrael. Sometimes Hashem gives reward to that generation as opposed to a certain number of years later on.