Isn't that how it should always be?
I'd imagine that if you're playing any co-op game, especially 4 player ones, if enemy #'s and difficulty don't increase with the increase of players than it would get boring very quickly, and someone's doing something wrong.
A recent example, I was playing the recent Red Dead Redemption Co-op missions. I was playing 4 players on "The Escape" I think, and we were all riding in a carriage. Only 2 enemies chased us at a time.
BOOOOOOOOOOOOOORRRRRRIIIIIIIIIINNNNNNNNNNNGGG!!!