Let me clarify, did you mean iPod Touch 4?
I'll break it down for you:
iPod Touch 4th is smaller and slimmer than iPhone 4s so iPod cases will not fit on the 4s.
Maybe you meant iPhone 4 and 4s cases? Most of the cases are compatible except for some alignment issues on the volume buttons and mute switch. Other than that, external dimensions are the same.
In face, I am still using all my iPhone 4 cases on my 4s.