Just got the bootloop issue.
Saw on youtube one guy say put in freezer - I was thinking WTF ?!
Might nothing to loose so I try - first put in fridge only (not freezer) for 15 mins. Doesn't help.
Then I try freezer for 15 mins.
And it works! My G4 managed to boot up !
Now quickly backing up my data cos I don't think this will permanently solve the problem.
Anyone encountering bootloop and haven't backup, worth giving it a try.
Make sure you put your phone in a zip log bag and wrap around your phone tightly without leaving too many air pockets.
Good Luck !