You may feel your device is hot to the touch in using.
But, this is normal and should not affect your device’s lifespan or performance.
As what the Samsung guide is claiming,
"Your device is not faulty and there is no issue with your device. ".
Below is the specification of Galaxy S4 including the explanation on why does this device is having a heating issue.
Galaxy S4 slim design
- 136.6 x 69.8 x 7.9mm
- Dual Quad Core (1.6GHz Quad Core + 1.2GHz Quad Core Processor)
1) All of mobile devices from Samsung have been observing the International Safety Specification(EN563).
2) And while charging, the device may heat up. But, Samsung device observe the International Safety Specification(EN563).
3) while using the device under the environment that ambient temperature is high, you may feel the device gets hotter than usual because your device work in the thermal equilibrium status with an ambient temperature.
But it is commonly seen not just on Samsung devices, but many other manufacturers devices when the device is used, and it is a general phenomenon of all electronic devices.