Restart the phone into Safe Mode and run it for a while. If you don't have any issues, and then you have a software problem and you will need to slowly load apps until you find the problem.
Posted via the Android Central App
I did a little test while playing a YouTube 720p video. I used an infrared the check the back of the G4. I started up the video and left the G4 in cradle.
Temperature readings:
0:00 77 degrees(just before video starts)
1:00 87 degrees
3:00 98 degrees
5:00 115 degrees
9:00 121 degrees (video ends here)
I picked up the G4 and it was really warm to my hand. It was a little scary because I've never held a smartphone that got this warm. (although all of my other phones are Samsung phones... Note 2/3/4/5 and S5/6... tested all of them side by side with the G4 and none of the Samsungs got to be more than 91 degrees when playing the same video)
Edit: after the video stopped, I left the G4 on the table for about 30 minutes. When I checked the temperature again, it went down to 76 degrees.
For reference, this is the YouTube video that I used for this test:
https://www.youtube.com/watch?v=LebPal5gKrc