Any chance someone had it repaired somewhere (i.e. not by Apple) - maybe had the back glass replaced by a 3rd party shop at sometime?
Dose the phone have siri (iPhone 4 would not)? Does the camera have face recognition and panoramo (again, iPhone 4 would not)?
If everything else checks out as iPhone 4s, then I'd guess the back glass was damaged and someone got it replaced.
In order to get it recognized by iTunes, you may need to plug it in and force it into recovery mode, see
http://support.apple.com/kb/HT1808 and then restore it as a new device.