When Mega Man reached the fourth Dark Man, he didn't want to fight against him (due to the fact that Mega Man thought he was the real Proto Man). Dark Man 4 used this to his advantage and fired one shot at Mega Man, severely damaging him and leaving him with one point of health left. Dark Man 4 slowly closes in on Mega Man after this, but before the final shot could be fired, the real Proto Man appears and blasts away Dark Man 4's disguise. Before leaving, Proto Man gives Mega Man an L Tank, fully restoring his health before the battle begins.

His stage theme here - https://www.youtube.com/watch?v=-FfwruMyFLg

based of the NES colors btw