Good day , I have a Samsung Galaxy note 4 SM-910T, the problem is that the motherboard is overheating when the phone is off as well as when the phone is on . A history behind it , the phone has gotten a little water damage (not fully submerged) the microphone stopped working. So I sent it to change the charging port, the guy botched the digitizer. So I decided to replace the parts 2 years after . Now when I installed the charging port and digitizer with the glass making sure all the connections are intact I'm getting the problem with overheating on the motherboard
I second KnightRalph316s opinion. The Note 4 was great in its time, but so many technological advances have come out since then. A newer phone would be a great investment no matter how affordable it is.
Also, there is no way any of us could know whats wrong. Anything could've happened when you replaced the parts. I'm not saying you were in the wrong. you were way out of warranty, and not worth official service. Just write it off as a cool experience.
Another also, the phone heats up when its off? thats hard to believe, unless its off and charging, in which case, I would open it back up carefully and re-solder the charging port .
I would highly suggest upgrading your phone to something else. But, if you're dedicated to prolonging the life of your Note 4, maybe changing out the battery would help. Or maybe replacing your motherboard if all else fails.