CASE 2: There exists a region of valence 5. Call it v.
11) As in step 6, fix a 4-coloring of the map M-v. By the same reasoning as in step 6, the neighbors of v must be four different colors, so now there are two possible unique types of colorings of the neighbors of v: (clockwise) ryygb and rygyb.
12) The proof that the ryygb case cannot exist is identical to CASE 1 above.