A sample would be good, and if possible, an identical shot from some other camera.

With my personal experience, every shot off the 4S has been better then the 4, (and I'm nowhere near as critical as Bruno, the 4, for being a thin phone in your pocket wasn't bad).

My initial thought with the little information here is possibly a defective 4S. Might be worth going to an Apple store, or somewhere else with a 4S on display and do direct comparisons. A lens out of alignment could possibly causing it, especially if the autofocus is going crazy.