After several years in the making, it's FINALLY complete!! PureSabe has taken Megaman 4 and practically rebuilt it from the ground up. Not content to just change the stages, music, boss AI and/or weapons, he has turned the game into one big tribute to the classic Megaman series; with tons of enemies, items, and bosses added in from almost every game in the series (and references to a few other games as well), there is plenty of fanservice to be had. Granted, this game, like many other hacks, is hard as balls, but like Rockman no Constancy and Grayzone before it, it's one of those hacks that's just so satisfying to play, you're not likely to care.
You can download the hack here: https://sites.google.com/site/rockman4mi/
(There's a link on Romhacking.net as well, but last I saw, that one has the last Cossack stage and the last two Wily stages still unfinished)