I just beaten Mega Man 4, Overall a solid game and well, Fairly difficult stages. The robot Masters in this one was mostly pretty easy if not too easy which made the Robot Master Rematch not that bad. Really the E Tanks in this is very easy to get and being able to replay levels and farm E tanks was not a hard task, especially on Skull Man's stage.
The music feels a bit weirder in this but I guess they were trying to go with a Russian aesthetic theme to it.

The story was also a lot more interesting and better than the other games, Dr. Cossack being not an evil person but just want his daughter back is a lot better of a villain story kinda deal for the NES at the time, and protoman being there was cool too.
Overall, Better than 3 but still behind 2.