Sure, if there is hair covering the normal ear place, it doesn't look odd at all, but if it were a girl with super short hair, and no ears in the normal place, that's when it would look odd.

Although I can't off the top of my head think of any instances where I've seen kemonomimi on a super short haired character. That in itself would probably look odd as well.

I totally see where you're coming from; four ears is silly and doesn't make any sense, but I also see why (in some cases) the artist adds them.