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Any iPhone 4 case made for the CDMA version will work. If it's Universal (meaning AT&T/Verizon) it will work.
 
Since it has the same design as the iPhone 4 CDMA model, I would imagine anything made for that model will work for the 4S.
 
If it's compatible with the Verizon iPhone 4, then yes. If it's compatible with both the AT&T and Verizon versions, then yes. If it's compatible ONLY with the AT&T version, then no. The reason is that the mute switch got moved down just barely enough for it to be an issue.
 
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