The fourth game in the Mega Man Classic series for the NES. It was released in Japan in December of 1991, closely followed by a North American release in January of 1992. PAL territories didn't get it until nearly a year later, in December of 1992. It has since seen numerous rereleases, including the Rockman Complete Works, Mega Man Anniversary Collection, Virtual Console, and PSN.It had been about a year since the events of Mega Man 3. Dr. Wily had seemingly perished after the final battle, having been apparently crushed by the falling debris from his fortress. But the peace is short-lived when Dr. Light is contacted by a new Mad Scientist named Dr. Cossack, who has sent his own new breed of robot masters out to challenge Mega Man. Tired of Dr. Light recieving all of the recognition for his accomplishments while his have been ignored, Cossack intends to destroy Light's crowning achievement and then put his broken body on display for the whole world to see, as proof that he is the superior one.However, once Mega Man had defeated Cossack's eight robot masters and confronted the doctor himself, it's revealed that Dr. Wily is still alive, and had kidnapped Dr. Cossack's daughter, Kalinka, and forced him to use his robots to battle his nemesis. Mega Man defeats Wily, who escapes, and peace is restored once more.Robot Masters:
A.I. Breaker: What makes Toad Man the Player Preference for first Robot Master is that to use his Always Accurate Rain Flush he needs to do a Rain Dance, however if he's attacked before his dance is complete, it'll completely negate the attack. With careful timing, it's a simple task to defeat Toad Man without him getting off a single attack.
Area of Effect: The Drill Bomb's explosion hits a larger area than the drill itself. Comes into effect against the second Wily Machine, which is weak against the explosion, but resistant against the drill itself, making for some tricky timing.
Bubbly Clouds: Subverted somewhat with Ring Man's stage, which begins in a cloudy tower but then goes into space and is filled with hippos and small floating Saturns. Played straight with the third Cossack stage, which takes place in the sky above the citadel and where platforms are often obscured by clouds.
Dual Boss: The Cockroach Twins from Cossack Citadel 3.
Dummied Out: Pharaoh Man's level was originally supposed to include a SphinxMini-Boss on the path to the Balloon Item. This boss did not make it to the final release, and the only data left of the Sphinx was the snake on his forehead, its chin, and some sort of sphere that Mega Man would have stood on during the battle.
King Mook: The Met Daddy, the boss of the first Wily stage.
Ledge Bats: This game introduces Up 'n Downs, an annoying little enemy that hides in Bottomless Pits and jumps out just as you're jumping over them, and can easily knock you in if you're not expecting them. Nearly every game after this has a variation of these guys.
Losing Your Head: Mummira, the mummy robots from Pharaoh Man's stage, attack by using their mummy bandages to throw their heads at Mega Man.
Noob Bridge: The Drill Bomb's remote detonation property (one of the reasons its one of the better "explosve" weapons in the series) is often overlooked (push B again before it hits its target). The 2nd Wily ship is basically a test to see if you know about it.
Offscreen Start Bonus: In the first stage of Dr. Cossack's fortress, there's a hidden E-tank buried in the snow to the left of your starting point.