It's easy, just follow the directions on this link. Read it a few times so you understand it.
http://forum.xda-developers.com/showthread.php?p=63627074
That's for a pc. Let me know if you have a Mac.
Just real quick. Download drivers for LG g4. Install on pc. Download the recovery image from that link.
Download adb and fastboot from here (install them system wide)
http://forum.xda-developers.com/showthread.php?t=2588979
Go into phone, settings, general tab, about, software, build (tap until it says you are a dev)
Back into settings, general, developer options.
Enable usb debugging
Click on enable oem unlock
Follow the commands in first link. You have to unlock your bootloader
Plug phone into computer, let it loaf drivers, make sure your USB connection is either mtp or ptp (doesn't matter which just not a media device)
Accept the key (message will be on your phone about connecting phone to computer)
Hit windows start key
Type 'cmd' and hit enter, this will bring up terminal.
Adb devices (will confirm device is communicating properly with computer)
Should show lg followed by serial numbers
Adb reboot bootloader
Phone will reboot to bootloader
Fastboot devices
Same thing with phone serial
Fastboot oem unlock
Fastboot reboot bootloader
Fastboot flash recovery twrp-2.8.7.1-h901.img
Fastboot boot recovery twrp-2.8.7.1-h901.img
Just please read the link, I typed this up for you, but it's late and I'm tired so double check for typos. Remember, I'm not responsible nor is anyone else. You are taking the risk in the event that something goes wrong.