What is 'the matter' to which the Pasuk is referring?
Ramban: It is referring to the promise that Hashem made to Avraham at the Bris bein ha'Besarim 1 - "Ki Ger Yih'yeh Zar'acha b'Eretz Lo lahem ... v'Dor Revi'i Yashuvu Heinah" (' ... that your offspring will be strangers in a land that is not theirs ... and the fourth generation will return here').
Why does most of this Pasuk merely repeat 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 their hands, but rather, due to the latter's 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