Mega Man 4

Cameo

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.

Golden cartridges

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.

Manga adaption

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.