Is it possible to upgrade the camera on an iPhone 4S? I'm a 4S holdout but would like to upgrade the camera (particularly the sensor). Is this possible to do? Is there an improved part that I can order and do the replacement myself?
If you are good enough to design custom cable from say iPhone 5/5S/6 camera sensor to logic board of iPhone 4S then provided there is drivers for that sensor in 4S iOS code it might work.

But no there is no off the shelf solution.