How big is your house? 4 cats need a lot of space. If they don't have a place to run/hide/relax, they may always fight. Especially since they aren't kittens.
My cats are both 18 pounds and one is definitely an alpha male. I lived with my sister for a while and her cats (10lbs each) are lucky they are small and fast because they got swatted every time they came within 2 feet of Bernie. I'm about 100% positive that Bernie could never get along with another aggressive cat. To adult humans, he's a huge baby, but to cats, he's the king. I guess its possible he thinks he's a human. He's so needy for attention, he gets accidentally kicked and stepped on often.