Both are basically the same appearance,hardware improvement lies mainly in the:
The processor,iPhone4 is using for 1 GHz single-core presenting the A4 processor,and Iphone4S is used with iPad is the same A5 dual-core processor,thus processor performance iPhone4 than of mononuclear A4 to faster,but in order not to loss of endurance Iphone4S use A5 processor was reduced to 800 MHz frequency to keep endurance.
In addition camera lenses do slight improvement,pixels from the Iphone4 5 million pixels to improve Iphone4s 8 million.
Buy apple 4 S money even the product itself several times