Camera Quality: Outdoors, Day Lighting
The iPhone 4S features a higher-resolution camera (8 MP to the iPhone 4's 5 MP), but image quality is also enhanced due to the addition of an IR filter and additional image processing logic built into the A5.
No Zoom, Outdoors, Noon, Full Sun
Pictures taken with the iPhone 4 often look better than what the 4S delivers, mostly because the older phone takes pictures at a cooler temperature, for which we have a natural bias.
With that aside, the iPhone 4S generally does a better job. It's more flexible under a wider range of lighting conditions and you enjoy the benefits higher pixel density when you view the pictures under a fixed desktop resolution. This effect becomes more evident in a macro shot, at the bottom of this page.
No Zoom, Outdoors, Noon, Shady
Full-zoom, Outdoors, Afternoon Sun