I've just run into this issue. One potential cause is what LG admitted to earlier this year, which I was unaware of until this problem happened to me just now.
http://www.androidauthority.com/lg-admits-g4-bootloop-problem-hardware-fault-669603/
There's apparently a loose contact somewhere on one of the components (I can't imagine it would be anywhere else but the motherboard). LG will fix it if you are under warranty. I, on the other hand, bought mine off of eBay and am not under warranty. So... if anyone with experience knows where the loose contact is and somehow can create a guide for this, that would mean the world to suckers like me who are left clueless and optionless.