While the garment may have more than four corners, the command is to to put tzitzit on four corners. Rashi connects this to the four expressions of redemption (just like the four cups at the seder).
on the corners of their garments: Corresponding to [the verse said in
connection with the exodus from Egypt]“I carried you on the wings
(כַּנְפֵי) of eagles” (Exod. 19:4). On the four corners, but not on a
garment of three or five [corners]. [This] corresponds to the four
expressions of redemption that were said in Egypt:“I will take you
out…I will save you…I will redeem you…I will take you” (Exod. 6:6-7).
- [Mid. Aggadah]
Chabad.org points out that
Any garment which has four or more corners is obligated in tzitzit. If
the garment has more than four corners, tzitzit fringes are only
attached to the four corners furthest from each other
Why Must a Tallit Have Four Fringed Corners? cites Rashi
Rabbi Shlomo Yitzchaki (Rashi) quotes a teaching3 that the corners of
the tallit are alluded to in a verse describing the Exodus: “I carried
you on the wings (כַּנְפֵי) of eagles.”4 The word kanaf, “wing,” can
also mean “corner.” As for why there are specifically four corners,
Rashi goes on to explain that they correspond to the four expressions
of redemption associated with the Exodus: “I will take you out . . . I
will save you . . . I will redeem you . . . I will take you . . .”5
Many say that this teaching comes from Rabbi Moshe Hadarshan, whom Rashi mentions earlier.