Why was it called Kiryas Arba?
Rashi #1: Arba was the name of the father of the three giants (Shishai, Achiman and Talmai - 15:14).
Rashi #2: It is based on the four [giants, including their father. The three] are called "Yelidei ha'Anak" (15:14).
Who was "ha'Adam ha'Gadol ba'Anakim"?
Rashi: It was the father of the three giants.
Rashi, citing a Midrash 1 : It was Avraham
Why was the land quiet from war?
Rashi: After seven years in which Yisrael conquered around them, the remaining Kena'anim were subdued and did not gather to fight them. Therefore, Yisrael began to divide the land.
Rashi, from Yalkut Shimoni Vayikra 587: In the merit that they honored Avraham (ha'Adam ha'Gadol ba'Anakim; they said "Nesi Elokim Atah b'Socheinu" - Bereishis 23:6), the land was quiet from war (Rashi - the 40 years that Yisrael were in the Midbar).
Radak: Yisrael slackened from dispossessing the remaining Kena'anim.
