What, exactly, did Hashem mean by, "the fourth generation"?
Ramban and Rashbam: Only upon the fourth generation of the sinning Cana'anim, would B'nei Yisrael return. It is the way of Hashem to "visit the sins of the fathers on the children, the grandchildren and the great-grandchildren," 3 to give them a chance 4 to do Teshuvah. 5
Since they would not return until four hundred years, the number of generations (of Yisrael) makes no difference. In fact, the four generations (of Cana'anim) plus the reason for the delay given in the Pasuk itself (their sin is not yet complete) explains why they had to wait four hundred years (Rashbam).
A proof that Teshuvah is applicable to Nochrim.
Why does the Torah mention specifically the Emori?
Ramban #1: They were the strongest of the Kena'ani nations.
Ramban #2: They were the first that Yisrael captured (Sichon and Og).
QUESTIONS ON RASHI
Rashi writes: "'The fourth generation shall return here' -Go and calculate Yaakov's generations
Riva #1; Mizrachi #1; Chizkuni: Yaakov was already elderly (age 130), and close to his demise, when he went down to Mitzrayim, so he is not counted.
Riva #2; Mizrachi #2: We are not counting the generations from the exile, but rather from when the actual servitude began, which was after Yaakov's death (Rashi to 47:28). 1
Gur Aryeh: "The fourth generation" means after going down to exile, i.e. not including Yaakov. 2
Sifsei Chachamim resolves this with Shemos 1:6, which connotes that the servitude began only later, after all Yaakov's sons died.
See Ohr ha'Chayim, who counts the four generations differently. Also, Malbim questions the assertion that Calev ben Chetzron is one and the same as Calev ben Yefuneh who entered Eretz Yisrael (see also Radak to Divrei ha'Yamim I 2:18, Malbim to ibid. v. 42), and gives other explanations of our Pasuk. (CS)