At the time of this writing, the Sprint version of the LG G4 should be getting an OTA rollout of Android 6.0 Marshmallow. Of course, the update will be pushed out in phases, so if you haven't received it yet, just be patient.
The update (software version LS991ZV9) actually started rolling out yesterday, December 17th. There's no word on when the update will start rolling out to other versions of the G4, but we don't expect it to be too long. Besides the usual new features and functions that Marshmallow has over Lollipop, there are some other enhancements including Sprint Global Roaming and more.
Don't forget to drop by our dedicated LG G4 section for further discussion: LG G4 Forum | Android Forum at DroidForums.net
Source: Sprint