The appearance betwee the iPhone 4S and iPhone 4 is almost identical except 4S comes with white version and 3 grams lighter than 4.

iPhone 4S has A5 chip, the same chip used on iPad 2, so the operating speed is slightly faster than iPhone 4 and better graphics display.

The camera on iPhone 4S has 8 megapixels while iPhone 4 only has 5 megapixels. The death grip is no longer a problem on iPhone 4S as Apple has fixed the antena problem. Therefore, you do not need to have protective case on 4S.

For more comparison, you can check here.