I'm pretty sure the 4S is a solid step up from the 4 as well. I think the 4S went to a 8MP sensor. I'm not expert and I know that MP aren't the end all, but everything I've read/seen points to the 4S being better than the 4. So to go from the 3GS to the 4S should be a notieceable jump.
*Edit: Added a possibly helpful link*
http://gizmodo.com/5852832/how-much-...oved/gallery/1