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.