Proximity Sensor Fix
I had an issue where the proximity sensor would detect when something got close to the screen during a call, but would not detect that the object had been pulled away, so I would have to hit the lock key or the home key to turn the screen back on during a call.
Fix: Spoke with T-mobile support. Seems the proximity sensor on the S4 is prone to getting fouled with dust. They suggested I spray compressed air in the ear piece. Low and behold my sensor works again but I have had to do it more than once. This seems to be a design flaw of some kind. Thanks Samsung...
If you are having issues with your proximity sensor give this a shot! It worked for me but may need to be done periodically.