Which level 4 monster is the best?

No, there’s no reason. For example, if silver pegasi and snipers are fighting (and they are lined up in the middle of the battlefield), then if the pegasi go first, the snipers won’t have time to shoot, and the outcome of the battle is clear. If the snipers go first, it’s still unclear who will win… But I’m rooting for the silver pegasi.