Using the autoroot kernel you can easily gain root access on your LG G4. This root kernel contains SuperSU along with busybox and init.d support. It's a stock kernel based on 20A software for the Android Marshmallow LG rolled out recently.
XDA forum member, cile381 who has released this autoroot kernel claims the kernel is neither tweaked nor contains any modification. It is a recovery flashable zip, so to flash this all you need is TWRP recovery on your G4. But note that this kernel supports the international version of the G4 bearing model number H815 only. Check out the following guide if you want to try out the root kernel:
How to root LG G4 on Android Marshmallow firmware 20A
Step 1: Download the autoroot kernel
Step 2: Connect your phone to the PC and copy the downloaded root kernel to the phone's storage and remember the location
Step 3: Disconnect the phone and boot into the recovery mode.
You might use Quick Boot app from the Play Store for this. Just download and install the app on your phone. Then launch it and follow the on-screen prompts to boot into recovery. Alternatively, use the following hardware key combinations:
- Switch off the phone
- Press and hold Volume Down and Power buttons together for a few seconds until you see the LG logo on the phone's screen
- Release the buttons for a second and then hold them back again
- The recovery mode menu should now appear
- Use the Volume Up/Down key to navigate between the option and Power button for highlighting
- You phone will now enter recovery mode
Step 4: In recovery mode create a backup of all data of the phone. From recovery tap on Backup, and then do a swipe on the bottom of the screen to confirm the process
Step 5: When the process is complete, install the autoroot kernel. Select Install then locate the folder where you have saved the root file. Select it and do a swipe on the swipe to confirm flash option at the bottom of the screen
Step 6: Now return to the recovery menu and select Reboot system to restart the phone
That's it. Your LG G4 should now be rooted on new Marshmallow firmware. More details about the root kernel is available on XDA development thread.