I use to do alot of sprite work myself. You could do alittle more inner shading on the ssj4 arms. Maybe give the tail more movment. Its just sitting there like a stiffy =x
Also next time you save a sprite sheet, make sure you *DONT* use any error diffusion. It totaly ruins your work and it gets pixelated very badly. 256 colors is for the best.
Good effort *thumbs up*