Can't get light off on back of iPhone 4?

If you are referring to the LED Flash on the back of the phone that is commonly caused by water damage. Check the charge port and see if you see a red line in it. Also check your headphone jack for a red dot. If red it is water damaged. Replacing the camera may work, however i have never seen it work. Also clean your board with rubbing alcohol and a tooth brush. Pull of all the EMI shields that you can (all but one) and rub and let dry to 10 minutes on one side and flip over and let it dry. This will clean off any corrosion that is on the board. DO NOT put it in rice, it will cause corrosion to build up and corrosion is what kills phones that have been working if they work after water damage.