Ok, so I've seen on this forum and other about the S4 heating up for some people. I have to say that I don't have this problem. On average my phone stays around 84 to 99 degrees at all times. (When not in direct sun light.) I can play 3D games and be on the net for 30 min or so and only get to around 104 degrees. (I have an Outterbox Commuter case on as well.) What I have noticed is when the screen brightness goes up, so does the temp.
I use a 3rd party battery monitor app for the temp. Its very useful and worth looking into.