T-Mobile LG G4 can be updated to Marshmallow right now using LG Bridge
Once you sync to your PC and check for an update, you should be given the opportunity to download and install Android 6.0. The update comes with Doze, a feature that increases the handset's stand-by time by putting the phone into a deep sleep state when it is inactive for a long period of time. Apps stop running in the background, and the bottom line is better overall battery life.
Android 6.0 also comes with a change to app permissions. Users will be able to allow or deny permissions on a feature-by-feature basis. And Google Now On Tap will return search results based on the content that is on the screen.
Remember, this is available for model number H811, which is the T-Mobile version of the LG G4. If you would rather the ease, familiarity and simplicity of an OTA update, you are welcome to wait for it to start rolling out via that method.
source: Reddit via AndroidCentral