I love my G4. However, that fact the my son has received Marshmallow on my old G3, while I am still waiting is very frustrating!
From what I've been reading, MM doesn't sound that special. Given my bad luck with poorly designed/implemented Verizon updates, I've disabled my update checker. About the only thing that would make me really want it is if they fixed the bug where the camera app resets itself like the data was cleared, and another where Play doesn't recognize a mobile data connection without clearing the app cache.
I don't want it either...just hoping when I do get it, it doesn't mess up my perfectly running phone.
Go into the application manager and swipe to the ALL tab. Scroll down and you'll see two apps named System Updates. One of them will show taking up space, the other will show zero bites used. The one showing zero can be disabled, which will prevent your phone from checking for updates.