It would be cool to create general Grevious's armor as a race or as armor either as 2 handed or 4 handed and i know it would be hard but hey its an idea

making a 4 armed version of him would be basically impossible, it's requires a new skeleton or and edited one to fit the body shape, then you would have to animate each arm to look different so that the to left ones and the two right ones dont move in sinc and look very unnatural.

the armor seems possible but would look out of place in the game, but then again so do some other mods.

Except for the fact that the player skeleton cannot be changed at all. The most one could do is have two extra 'dead' arms, just like Alexander's Wings. If this is to be done, it will have to be done in the two armed version, and it will be, at best, a poor representation as it will look just like a normal human/elf/beast. In short, it isn't very plausible.