Do you have multiple litter boxes for the 4 kitties?
I only have two cats, and one litter box. Which hasn't been a problem up until a week ago when our older kitty started using places other than the litter box to do her business. We can't figure out why she's started peeing around the house. We've taken an extra effort to make sure the litter box is as fresh and clean as we can maintain it, and we've even tried switching the kind of litter we use.
Have you ever had this problem? Do you have any advice on how to fix this?
Yes & yes, I had that problem!! I bought a new litter because I couldn’t find their regular and they needed some immediately. They hated the new litter and decided to do their business on the rug in front of the little box instead. If you switched their litter before this happened that might be what caused it, other wise I’m no expert on this kind of stuff. This article might be of help, it has lots of ideas. Good luck, hope she stops soon. :)