I had the same issue for quite a while ... I believe I found that it's the "Knock On" feature of the phone causing the problem. If the LG G4 is tapped twice while sleeping, it wakes. On the LG G3 that setting could be turned on/off, but on the LG G4, that setting is not made available.
I think that the accelerometer in the phone, or something in that '''Knock On''' feature that monitors the accelerometer, has gotten way too sensitive in one of the phone updates. Now that I've turned off that feature, ''my phone stays dark and quite'', as it should after I put it to sleep.
There's an app in Play Store called '''QuickShortcutMaker''' that can be used to make a shortcut to the hidden Knock On setting. After installing, while looking at "Activities", type "Knock On" in search. Settings should show up in the list underneath. Select the down-arrow next to Settings, and a couple sub-settings will appear. They should be "Knock On" type settings and you can choose either one, they both do the same thing. Select one of them, then change the name from Settings to "Knock On" for the shortcut you're going to create, and you can choose a different Icon for it. At the bottom, create the short cut, then find it on your display and select it. Or just select the "Try" in the QuickShortcutMaker to open the Knock On setting and disable it.
Here's a link to where I found this information: https://www.phonearena.com/news/How-to-disable-KnockON-double-tap-to-wake-on-the-LG-G4-LG-V10-other-LG-smartphones_id75888
LG G4, may you sleep long until needed.
Hope this helps!
BabbleBits