This is the famous "four-color problem" of map coloring.

The content of the four-color problem is: "Any map with only four colors can make countries with the same border have different colors." Expressed in mathematical language, it means "divide the plane into non-overlapping areas at will, and each area can always be marked with one of the four numbers 1, 2, 3 and 4, without making two adjacent areas get the same number."