Reports are coming in from users of Samsungs’ new flagship Galaxy S4 that the device is getting a little too warm for comfort.
It seems users of the Exynos Octa chipset variety are experiencing overheating issues, which is having a massive impact on battery life.
One user even went as far tracking the temperature on his Galaxy S4; getting 27°C when idle, 35°C when browsing or checking email, and 40°C when playing games!
Some are theorising it’s to do with the location of the Exynos Processor, being too close to the  radio and battery, compounding the heating problem.  Whatever the cause is, most are recommending you turn off bluetooth and GPS (unless currently being used), dim the screen, and put the phone into power saving mode to restrict the processor.
Hopefully Samsung will have a proper solution to this soon and we’ll update this post if we hear anything.
Source: XDA Developers Forum