From what i've seen the 4S camera has better tonality, little better dynamic range, less saturation, more detail (obviously), but larger noise grain size (I don't like).
here is a direct comparison:
iPhone 4:
http://www.xti9.com/pics/iphone/4ng.jpg
iPhone 4S:
http://www.xti9.com/pics/iphone/4Sng.jpg
here is a direct comparison:
iPhone 4:
http://www.xti9.com/pics/iphone/4ng.jpg
iPhone 4S:
http://www.xti9.com/pics/iphone/4Sng.jpg