Here's couple of suggestions that I found online:
- If you’re using a Micro SD card in your Note 4, then that’s the
first thing to check. Remove the card from your phone and test to see if
the random reboots still occur. If the reboots have stopped then back
up the data from the card on your computer and try reformatting it on
the Note 4 via Settings > Storage > Format SD card.
- You could try a factory reset. Make sure you have all your data backed up. Go to Settings > Backup and reset > Factory data reset > Reset device > Erase everything.
Restart your phone in safe mode if it doesn't do it anymore then it is more than likely an app that's causing the problem.