My S4 started running very hot and draining the battery after I moved a few apps and a bunch of photos and video to the SD card. I removed the apps but the problem continued. I tried the factory reset and that didn't help. When I unmount the SD card, the battery consumption and temperature are normal. Now I only mount the SD card to show photos or something, and the S4 immediately heats up. I have not yet tried to reformat the card or a different card.
I suspect that the OS is continuously reading the card when it is mounted, and the problem shows up when you add more content to the card. It is almost surely a software issue.
Those of you with the heat/battery drain issue, please try unmounting your SD card to see if the problem resolves. Yup, it's a workaround, but until the fault is found, it can't get fixed.
Regards,
- Just Gary