(I got the idea from this thread: https://forum.xda-developers.com/g4/...-loop-t3311623 )
After reading the first few pages of the thread linked above, and seeing the solutions posted by some about putting some sort of "pressure" material between MB and camera, I decided to try something easier before I resorted to disassembling/baking. This "bootloop" issue started on my phone 2 days ago, and I ordered a new phone, but I wanted to do a backup so I only needed my G4 working for a few minutes.
I removed the back cover, removed and re-inserted the battery. Then leaving the cover off, I held finger pressure (with two fingers) on the plastic on either side of the volume up button. I then powered the phone on (maintaining pressure), and was pleasantly surprised that the phone successfully booted!!
Continuing to hold pressure with the two fingers (sort of holding the sides of the phone with thumb and another finger), I was able to swipe to unlock, select menus, enter settings, and perform a full backup to my SD card!! I am very happy!
I may do the bake thing later if I can't restore my stuff to the new phone from the SD card, but for now, I'm just happy it worked. If all you need is a quick backup, give this a try first. Good luck!