The only physical change to the 4S from the 4 is that the volume buttons were moved slightly, so as long as it is not an older case that has individual holes for the volume buttons, you're fine. There is no reason to buy a case that is only compatible with the 4S as that case would also be compatible with the 4. I hope that answers your question, unless you have a specific reason that you want a 4S only case, which I am fairly certain does not exist.