The back keys were first used on the LG G2 and since then you could quickly launch the camera by pressing the volume down key on the back twice. But on the LG G4 the Quick Shot feature is slightly different, as double pressing the volume down key doesn't just the camera as quick as possible, but it takes a picture as well (as soon as the camera app launches).
If this feature doesn't find you well and you want to only launch the camera application without taking instant shot (like the old days), do as follows to disable quick camera shot on LG G4:
1. Open the Settings application and go to the General tab
2. Tap to open the Shortcut Key settings.
3. To completely disable quick launch camera, simply turn the Shortcut Key off, by just sliding the button
4. To disable the option to take picture as soon as the camera application launches (by pressing the volume down key twice), simple unmark the Quick Shot option