[Galaxy Note 4 Q&A, Help & Troubleshooting] quicksite:
Galaxy Note 4 - Endless boot loop

I've experienced this 2 years ago and you've probably googled around, others have too. 2 years ago when this happened to me, and I first tried replacing the battery, with no luck, I sent to Samsung to diagnose. Their claim was that it had a failing motherboard. but the price quoted to replace was outrageous, iirc, over $300. I passed. And then bought a "new" used Note 4 from Swappa. The first had the same problem and I got it refunded. The second was scratched up on lens etc and I sent that back. Upon recommendation of a high-volume phone reseller on Swappa (I'd spoken to the owner, who was not a bull**** artist, but actually knew this phone very well, and conversation demonstrated to me that he knew the whole cycle of Android phones, with the Note 4 reknown for its usability and features. His own advice was that it really would be a matter of time before any used replacement for the Note 4 would run into this bootload loop problem, and he strongly recommended that I spend the same amount of money or less (Swappa/used) and get a newer-aged phone. He rec'd the original year Pixel XL, and I was not fond of the whole idea of closed off back, no microSD, and untold problems I'd had over the years in getting big video files off my Note 4— and how I'd relied upon the USB cable. Though I spent a week googling the pros and cons, ultimately against my intuition and experience with many android phones over the years, I decided that maybe the guy was right, and the Pixel was said to have a better camera and video recording than the Note. So I purchased a used Pixel XL and it was virtually pristine and running Pie. I gripingly adapted to all the problems, but the phoine was for sure more reliable than my aging Note 4s.

But after 6 months I couldnt take it anymore, with Google's f'd up denying ability to record phone calls and just other nuisances, with a far inferior Notification Shade system than my Note 4's with running Good Lock. So I once again went to swappa and took the plunge again on a Note 4.

Have been using for about 6 months and I am only now running into battery degradation— but NOT the bootload loop problem. And I just spent 2+ frustrating hours on Amazon comparing Note 4 replacement batteries, annoyed that all the most highly rated Note 4 batteries (Powerbear, RavPower, etc) did not have NFC compatability, therefore no Google Pay. I came here to XDA to see which battery people rec'd and when I then went back to Amazon, the reviews were FILLED with "worked great for a month" or 4 months, then died. But meanwhile, in reading toins of reviews, I came across people describing how their Note 4's had been starting to die -- but they got xx-brand-name battery and voila, it worked great again. Followed by people saying, "don't buy, it's trash".

Moral of the story is I do believe there is life-use wear and tear on the old 2014 Note 4's, and in your case it may indeed be a motherboard failure. When I first began researching these problems 2 years ago, I found that the MOST USEFUL AND KNOWLEDGABLE INFORMATION anywhere on the web was at the site TheAndroidGuy.com — If you go there and search, you will find a comprehensive IF THIS, then TRY THIS article, which has multiple permutations. The article may give you hope, but more likely it will conclude that it's a hardware problem afterall.

This Note 4 I currently have is so much better than the Pixel XL in usability and features, so I am looking to milk all I can from this model, and am hoping that in my case a new battery might indeed help. But I also am sanguine enough to recognize it may not last. Hope this was helpful in some way by some realistic data.