Why, the 4S is identical physically speaking to the 4. Doesn't seem to make sense produce a 4S case because it will work on the 4.
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.
...and that's precisely what some did which is why they had cases for the at&t IP4 and cases for the VZW IP4.it would be silly for case makers to make two different cases for something so insignificant.
...and that's precisely what some did which is why they had cases for the at&t IP4 and cases for the VZW IP4.