8) If that submap does not contain g, then, within that submap, recolor all red regions green and all green regions red. We now have a 4-coloring of M-v in which no neighbor of v is red. Therefore coloring v red results in a 4-coloring of M. Contradiction.
9) The contradiction in step 8 shows that the submap in step 7 must contain g. Therefore there exists an alternating path of red and green regions that divides M-v into two parts such that y is in one and b is in the other. See figure 1.
10) Within the part containing y, recolor all yellow regions blue and all blue regions yellow. We now have a new 4-coloring of M-v in which no neighbor of v is yellow. Therefore coloring v yellow results in a 4-coloring of M. Also a contradiction. CASE 1 leads to a contradiction either way, so it must be false.