HTC One M8 Charging Issues
The charging issue may be caused thanks to some app which is running on background and a lot burdens CPU. 

Try to kill all apps before/while charging to decrease CPU consumption 


That 4 hours of nonstop usage are normal I think. If you were playing games or something like that you should be happy for 4 hours. :) So answer is: Yes, It's normal.

Look at this guide