Wanted to get pics of 4, 4S and the replacement before posting, sorry.
The back panel I bought for my 4S -DOES NOT- fit!
of course it will fit an iPhone 4, but not a 4S.
Also, as you may have read, cover-cases that have EXACT cutouts for the mute switch and volume buttons will not fit either. I couldn't take a picture, but stood them up next to each other and the mute switch is in quite a different spot. Volume buttons were only very slightly off. A case that has a wide, open area (without exact cutouts) will probably fit.
Also note: This two original iPhones 4 and 4S are from Japan's Softbank carrier, most likely the same as AT&T's carrier model.
There is another carrier in Japan, KDDI/Au, which carries the CDMA version, however I have yet to see one.