[Yumetaro]

I was thinking of playing around with Mega Man 4, but is there a good enough amount of space to work with? Or are there other Mega Man games with more space?  I would seriously love to know!

[Midna]

Rockman 4 MI is really the only (really big) MM4 hack I know of.

For the other NES games, there are a handful of MM1 hacks, quite a bit of MM2 hacks, a few MM3, a sizeable amount of MM5, and basically none at all for MM6.

[OneCrudeDude]

If you want to know if people are going to make ROM hacks that could mirror the likes of Mega Man 9 or the various fangames, then no.  The closest to that would be Mega Man Forever, itself being incomplete for god knows why.  For the time being, until the "skilled Mega Man people" who make the graphics and music decide to approach an NES hacker, prepare for the NES games being MM2 hacks where Air Man now behaves like Metal Man while Kirby music plays in the background.

[Trax]

A part of the hacking process is to find out about this information. Take a look at your ROM in a hex editor, and check for ranges of data that consist only of the value 0xFF. Usually at the end of a bank. You'll have a good idea of what kind of unused space the original ROM. I say "original" because you can expand a ROM, but it's not always trivial, especially for NES ROMs...

[Matrixz]

Compared to MM4, Mega Man 5 has more space in some elements of level editing, including scrolling data, screen presets and maximum screens that a level can be in length. So basically, you can make longer levels.

Besides, it's easier to deal with enemy graphics in MM5 (automatic, instant swapping of enemy CHR banks), and it has more features than MM4 for doing graphics effects in a level, like swapping or animating palettes and CHR.