> Michigan, with its separate upper peninsula, doesn’t actually prevent the U.S. map from being four-colorable, but it could, mathematically.

Couldn't you always "connect" the upper and lower parts of Michigan, four color the map that way, and then erase the line you drew? I'm having trouble seeing why Michigan (or Michigan-like states) could ever pose a problem as it seems to be solvable easily.

Does the hole caused by Lake Michigan pose a problem? I see the number of colors grows if you have a torus, but Lake Michigan (or its equivalent) is a 2D hole not a 3D one.