I used to have the same issue. I'm down to 1 cat now. (2 were my ex's and 1 of mine went kamikaze off his balcony and disappeared)
I don't know how much space you have, but we had to get 4 kennel crates, a litter box/food/water, toys, and bedding for each. Lock each cat in the crate for a couple of hours when they're napping or not wound up - long enough to get their scent and use their litter box so they can establish it as their own. This took care of the problem for us. Each cat then has their private space and when they wanted to be left alone, they went to their space.