Identify Iphone model, 4 or 4S?!!

I cant identify my iphone. I'm confused about that my iphone is an 4 or 4S. On the back cover, the model number is A1387, which means 4S. But the model number in options is "MC318LL/A", which indicates 4. It has no panorama camera function either. Bought this iphone used. Is it possible that the previous owner has changed the back cover? please help