I just noticed this morning that there seem to be two principle uses of the phrase “four corners” in the scriptures. First is the gathering of Israel. They’ll be scattered to and gathered from “the four corners of the earth” (e.g., Isa. 11:12). Second is the altar of sacrifice in the tabernacle (and later the temple). The Lord instructed the Israelites to create horns on “the four corners of [the altar]” (Ex. 27:2; 38:2) as well as on the altar of incense.
This makes me wonder if the altar represents, in part, the earth. It also makes me wonder if the ashes from the burnt offerings are to remind us of Israel, and the prophecies that she would be scattered and gathered. This seems reinforced by the fact that Nephi quotes these prophecies of the scattering in the same passage that he compares Israel to being “burned … as stubble” (1 Ne. 22:15).