I have many times confused with this and my friends gave me different types of answers but i am not satisfied with it.
How can one differentiate iphone 4 and iphone 4s by looking its physical appearance?
The iPhone 4S and iPhone 4 models all use the same general all stainless steel body design and a thin "metal band" that wraps around the sides of each phone. However, the iPhone 4S and iPhone 4 (GSM) have an access panel on the right side (and an enclosed Micro SIM card) and the iPhone 4 (CDMA) does not have an access panel or a SIM card.
Additionally, the iPhone 4 (GSM) has three antenna "breaks" in the metal band (one on the top and one on each side toward the bottom) and the iPhone 4 (CDMA) and iPhone 4S have four (two on both sides toward the top and bottom).
The notches on the stainless steel band are different.
On the 4 there is one the top & 2 on the bottom sides, whereas on the 4S they are 4 on near each corner.
May not notice this one unless you holding both side by side but the volume buttons, silent switch & headphone jack are slightly repositioned as well.