Firstly there are ONLY TWO differences between the 4 and the 4s
1) It is the A5 processor in contrast to the A4 processor, which in my knowledge is much faster. However, in iPhone 4 the A4 processor works perfectly fine without any lag..-according to what i my usage-
2) The camera, without a doubt, the camera on 4s is much better compared to the 4.. but in comparison to other smartphones on the market the iPhone 4 takes pretty decent photos and have fast shot to shot speed. Although the camera have a difference between 5 and 8 MP but, lets be honest unless you are a professional, which i doubt they would use an Iphone, you do not need a 8 MP camera.. The 5MP camera on the iPhone 4 is also quite decent and can take images without losing much detail..
In conclusion, there is not really much noticeable difference with both the 4 and 4s even though apple claims that on the inside it is totally difference. If you are using both phone with out anyone telling you one of which is the 4s you wont even notice the difference.