The peak of 2D Mega Man, in my opinion. This game strikes the perfect balance between style and control - even the simplest actions look and feel awesome. Creative level design, decent challenge without being unfair, and a great soundtrack. It's a refinement of everything that came before - a zenith for Mega Man.
Special praise should go to this game's sprite work. Incredibly detailed pixel animation is tremendously difficult and time-consuming, and X4 is another entry in Capcom's dominance in that field during the 1990s.
Yes, the English voice acting is atrocious. The Japanese VA is pretty solid, though.