From Wikipedia: “The four color theorem, on vertex coloring of planar graphs, is equivalent to the statement that every bridgeless 3-regular planar graph is of class one (Tait 1880). This statement is now known to be true, due to the proof of the four color theorem by Appel & Haken (1976).”

Here is a simple implementation of this equivalence, that converts a map from “4-face-colored” to “3-edge-colored”.

Here is the link to try the new functionality: https://github.com/stefanutti/maps-coloring-java/releases/download/4ct/ct-ui-swixml-2.3-jar-with-dependencies.jar