Apple iPhone's are so similar as they work .The iPhone series like iPhone 3G and iPhone 3Gs are look same and iPhone 4 and iPhone 4S are also look same but it has minor difference an normal person cant find easily .Coming to the iPhone 4 and iPhone 4S there are some tiny differences in case but large difference in there hardware.
1.Left and top side Black line
2.Siri
One simple way to identify the iPhone 4S and iPhone 4 models, as well as all other iPhone models, is by the model number printed in small type on the back of the phone. The model number printed on the back of the GSM-equipped iPhone 3GS is A1303,
The model number printed on the back of the GSM-equipped iPhone 4 is A1332,
The model number on the back of the CDMA-equipped iPhone 4 is A1349,
The model number on the back of the iPhone 4S is A1387.
1.Left and top side Black line
2.Siri
One simple way to identify the iPhone 4S and iPhone 4 models, as well as all other iPhone models, is by the model number printed in small type on the back of the phone. The model number printed on the back of the GSM-equipped iPhone 3GS is A1303,
The model number printed on the back of the GSM-equipped iPhone 4 is A1332,
The model number on the back of the CDMA-equipped iPhone 4 is A1349,
The model number on the back of the iPhone 4S is A1387.