Mega Man 4, known in Japan as Rockman 4: A New Evil Ambition! (ロックマン4 新たなる野望！！ Rokkuman 4 Aratanaru Yabou!!?), is the fourth game in the Mega Man series released for the Famicom and NES late in the console's life (1991-93). The game introduced the New Mega Buster as well as the concept of a "fake" villain (Dr. Cossack).

"Mega Man shattered Dr. Wily's plans three times and world peace has been maintained so far... but history repeats itself. Dr. Cossack, a mysterious scientist, has invented eight powerful robots and sent them after Mega Man. Mega Man starts for the battle again, this time equipped with the powerful New Mega Buster!!"

―Intro for Mega Man 4

Mega Man 4 was the first game in which it was not known that Dr. Wily was the villain, using a new antagonist until the end, Dr. Cossack. The game also introduced the New Mega Buster, an upgraded version of Mega Man's Mega Buster. This upgrade allows him to charge his shots for more damage. Eddie, a robot that gives Mega Man a random item, was also introduced.