In Mega Man 4, Dr. Wily is declared as having been killed by the explosion of Gamma. A year passes and Rock, Roll, and Dr. Light are living in peace. But soon, the peace is broken by a new threat.
A supposed colleague of Dr. Light's, the mysterious Dr. Cossack, has sent out eight of his own Robot Masters to find Mega Man, whom Cossack blames for the kidnapping of his daughter. Mega Man, hoping to clear his name, sets out to find Dr. Cossack, this time armed with a powerful new weapon: the Mega Buster. Mega Man must now battle with Cossack's fiendish Robot Masters and find Cossack's daughter before it's too late.
Gameplay-wise, this installment is similar to the previous game in the series. Like before, Mega Man has to complete eight stages, defeating their robotic bosses and acquiring their weapons as a reward. He retains his abilities to jump, shoot, climb, and slide. His dog can be transformed to access different area types. A new addition is the Mega Buster weapon, which can be charged to produce powerful shots.
The "antagonist" of Rockman 4, Dr. Cossack, makes a guest appearance in Rockman 8. But wait! Before you go screaming about "I already beat that game! Where is he!?", he doesn't appear in the game, but in a manga series BASED on Rockman 8 that was released in Japan! Cossack's daughter, Kallinka, also makes a quick appearance in the manga.
During production Capcom held a competition to design a boss monster for Rockman 4. From 70.000 entries, eight winners were selected. Those winners were not only rewarded with their bosses in-game, but also sent a golden cartridge of the game. This cartridge colour was not reproduced, so most likely only eight modules in that colour exist.
In Japan, a doujinshi manga (fan-created manga) of Rockman 4 is available. It follows the game's plot more or less straight through, but it's missing two boss encounters from the game: one from Cossack's citadel (the Wall Walkers), and one from Wily's castle. The manga was published by the manga label BomBom, and it's a rather rare find.