Mega Man 4 takes place in an unspecified year during the 21st century, described as the year "20XX". One year after the events of Mega Man 3, a mysterious Russian scientist, Dr. Cossack, unleashes an army of robots with the intention of world domination, much like Dr. Wily before him. Concerned over the situation, Dr. Light calls upon his own greatest creation, the hero Mega Man, to go after Cossack's Robot Masters, who have seized control of eight cities.[7] He also creates the support robot Eddie and equips Mega Man with the New Mega Buster, an upgrade for his Mega Buster.[8]
Upon defeating the eight Robot Masters — Toad Man, Bright Man, Pharaoh Man, Ring Man, Dust Man, Skull Man, Dive Man, and Drill Man — Mega Man makes his way to Cossack's icy fortress. However, in the middle of his battle with Cossack, Mega Man's brother Proto Man teleports in with Cossack's daughter, Kalinka. The girl begs Mega Man to stop fighting and explained that Dr. Wily had kidnapped her, forcing her father to build an army of robots and antagonize Mega Man as ransom.[9] With Wily's plan undone by Proto Man, he steps out of the shadows, furious for his betrayal.[10] Mega Man pursues his nemesis and fights through the scientist's Wily Castle, but Wily manages to escape in the end. Mega Man escapes as the fortress begins to self-destruct, and rides home on the top of a passing train, where he is greeted by Roll and Rush.
