When Google released the Marshmallow source code, many manufacturers were quick to jump on the news and start announcing updates to their flagships. In mid-October, LG started rolling out the android OTA in Poland first as a test, with promises to expand to other countries and operators later.
Now that update is ready for the US and Sprint is the first operator to get it. On its support page for the device, Sprint says the update was available yesterday, December 17th. It's rolling out as software version LS991ZV9 with Marshmallow and Sprint Global Roaming. There'll be a lot waiting for you when you jump from Lollipop 5.1 to Marshmallow 6.0, including app permissions, the new floating text selection toolbar with options to expand it through Google Translate or Wikipedia, better external storage integration that should be handy with the G4's MicroSD slot, and so much more.
If the OTA update hasn't appeared on your Sprint G4 yet, you can start hammering the About screen while checking for updates. There's no guarantee it'll come sooner to you, but it might relieve your stress.