What is the 'thing' to which the Pasuk is referring?
Ramban: It is referring to the promise that Hashem made to Avraham at the B'ris bein ha'Besarim 1 - "Ki Ger Yih'yeh Zar'acha be'Eretz Lo lahem ... ve'Dor Revi'i Yashuvu Heinah" (' ... that your offspring will be strangers in a land that is not their's ... and the fourth generation will return there').
The bulk of this Pasuk seems to be a repetition of Pasuk 4?
Seforno: What the Torah is now saying is that a. it is not on the merit of Yisrael that Hashem miraculously delivered the Cana'anim into our hands but because, due to their sins, they deserved to suffer defeat at our hands 1 and not at His, and b. that we should earn the right to inherit the land by carrying out the fulfillment of Hashem's promise to Avraham Avinu. 2