T-Mobile started updating the G4 with Marshmallow just last week, and now it's the AT&T variant's turn. The OTA is rolling out to the G4 on AT&T (H810) over the coming days. You'll have to get onto WiFi, though. This is a big one.
AT&T's changelog for this update is not terribly informative, but here's what we've got.
- Android OS 6.0 Marshmallow
- Video Calling
So... Marshmallow includes a lot of things, most of which we can work out on our own. LG's UI isn't changing much in 6.0, but the phone will gain Doze mode, adoptable storage, and refined do-not-disturb controls. The carrier-specific video calling is nice too, assuming anyone ever actually uses it.
The update clocks in at 917MB and will boost the build number to MRA58K (6.0). You can head to the update menu to trigger the download, but you'll also get a notification when the update is available for your device.