I'm inclined to accept Phil's answer, but there is a problem with his answer; namely, if the "four generations" are to be understood as "four hundred years" then why is the bible not speaking of the "fifth generation" returning? since only after the four hundred years of sojourn in Egypt did the Israelites return to the land of Canaan, why speak of a fourth generation returning, when in fact they were still in Egyptian bondage?
I want to suggest that the four generations are to be taken literally. We can make this work only if we reckon the four generations from the time Abraham's seed was first born in the land of Egypt. So Jacob and Levi would not be considered part of the "four generations" since they were born prior to the migration to Egypt. Kehat then would be considered the first generation to be born in Egypt. If my theory is plausible then we have: Kehat - Amram - Moses - fourth generation which made it back to the land of Canaan. Keep in mind that not only did Moses die before entering Canaan, but the entire third generation (besides for Caleb and Joshua) was wiped out in the desert during the forty year wandering in the desert, thus the fourth generation was the first to actually enter the land of Canaan.
I'm aware that this raises new problems in regards to the "four hundred year" prophecy, since it's not possible to squeeze in four hundred years in a tight three (literal) generations lifespan. This has been dealt with somewhere else. It is not in my interest to get involved in this complex problem, but all I can say is that this does not in any way undermine the premise that the four generations are four literal generations, as common sense supports; if anything, we could say that the four hundred years is modeled after the four generations and thus should not be taken literally. The motif of punishing the fourth generation (in this case the Amorites) is repeated many times in the bible, and is even found in the ten commandments. Thus it is plausible that the four hundred years prophecy is built on that motif.