The Four Corners of Civilization is the "known world" of Temerant.
Depicted on the map at the beginning of both books, the Four Corners consists of seven countries all greatly differing from each other in culture. Judging from the map, the name describes the four largest nations, territory wise. These consist of the Commonwealth in the south-west, Ceald in the north-west, Modeg in the north-east, and Vintas in the south-east. These frame the Aturan Empire, with two smaller outlying nations the Small Kingdoms and Yll. The map also features the country of Ademre which is not part of the Four Corners. The Four Corners is bordered on three sides by water and the Stormwal Mountains on the other.
In The Chronicle
The Four Corners is the setting of The Kingkiller Chronicle, wherein the main character, Kvothe, discusses his childhood in The Commonwealth and his later travels to Vintas and Ademre. The Commonwealth became known as such sometime after the fall of the Ergen Empire. The Four Corners currently seems to share its geographical space with a parallel world known as The Fae. Whether this universe encompasses all of Temerant remains to be seen, but doors to the microcosm can be found throughout the Four Corners.