My G4 bootlooped 2 days ago, out of the blue. It's a Verizon VS986, the box says it was built in May 2015, I believe the serial number starts with 505. It's hardware rev 1.0.
It rebooted and kept doing that continuously. Pulling the battery and trying again was no different. I then pulled the battery for about 5 minutes, and that time it ran successfully for maybe a minute, and was at the "Optimizing apps" screen, before it started rebooting again.
I did some reading, and put it in the freezer (with the battery removed) for a few hours. Put the battery back in, and put the phone in a ziplock bag. This was to avoid getting condensation on the phone itself with it being out of the freezer, and in a warm environment. I put it back in the freezer, and was able to get it to boot up fully. After that I was able to back up my data, fortunately. I put the phone between 2 ice packs while doing the lengthy backup process, to keep it cool. The phone was still in the bag, again to avoid condensation on the phone.
I took it to Verizon the next morning, they are having a replacement shipped to me overnight. I did not have to use my phone insurance to get the replacement, fortunately, since they inspected the phone and found no physical or water damage. I bought the phone 14 months ago.
I read here today that going through LG for the replacement may be better than going through the carrier, but hopefully this will work out OK for me. I hadn't read that yet, my assumption was starting with Verizon would be better, and then go to LG if Verizon wouldn't help me.
I'm very glad that I was able to get it running long enough to copy my data off it, at least. I hope the replacement is reliable. I hadn't upgraded to Marshmallow yet (I had kept mine on 611A, not rooted), so I guess the replacement will be a chance to try the new OS version.