THE BIG RED GUY:
Seems he is portrayed from a higher point of view than the others.
You might want to move the light from the left to the front, this way some parts are too dark. Mouth color doesn't make sense and is too similar to the highlight color.
THE BLACK GUY:
I see nothing wrong with him. He's tops!
THE TWO OTHER GUYS:
Not really sure what they are supposed to be? Are they glowing? If so, then the shadow makes no sense. How will they look on a background other than white?
Maybe turn the hands upwards to make them more defined against the body.
Hope this helps.