Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
It's a digital zoom, it'll never be good. It's no different that taking a picture, then cropping it.
 
The worst thing about it on the 4S is that I find the camera has a very difficult time properly focussing / exposing on things when zoomed. This goes beyond a simple loss of resolution and becomes a question of full-on imaging capabilities in that mode.
 
Why don't you just leave it unzoomed then crop it later? The end result is the same with potentially better results.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.