I'm still rocking my iphone 4. It was so well made that I haven't had reason to replace it. Checking out the specs on the 4s, I really like the upgraded camera in that model. I can't see much other reason to upgrade as the iphone 4 does everything I need it to do!
I had an iPhone 4s but recently upgraded to the 5c. I loved my 4s but the 5c just kicks the pants off my old phone. It has a better camera, way better processor and a larger screen (although only slightly). In my view, newer is better, at least in terms of iPhones.
Battery life is not brilliant but there is a lot running all the time -as long as you charge every few days which is not a problem for me -I can only say good about apple both in their stores and on the phones -they help years after you bought the product for free -they exchange if faulty -friend had her 18 month i pad replaced -not under warranty.