Proper noun
four color theorem
(graph theory) A theorem stating that given any separation of a plane into contiguous regions, producing a figure called a map, no more than four colors are required to color the regions so that no two adjacent regions have the same color (adjacent being defined as two regions sharing a boundary, not counting corners, in which three or more regions share a boundary).
Another theorem of this type is the Four color theorem whose computer generated proof is too long for a human to read. Source: Internet
Four color theorem main A four-coloring of a map of the states of the United States (ignoring lakes). Source: Internet