I see you mentioned the unit cannot turn on anymore. Are you able to get the unit in download mode?
[https://www.androidexplained.com/lg-g4-how-to-boot-into-download-mode/|Courtesy of this site.]
LG G4 Download Mode
* Download and Install LG’s USB Drivers
* Plug the USB Cable into the PC(not connected to the LG G4)
* Have you PC Powered On and Logged in
* Power Down the LG G4
* Press and Hold the Volume Up Button on the LG G4
* While Holding Volume Up, Plug the USB Cable into the LG G4
* Continue Holding the Volume Up Button After You See ‘Download Mode’
* Let Go of the Volume Up Button When You See the ‘Firmware Update’ Screen on the LG G4
The connection would either be the phone not turning on or the driver.
Hope this helps.