LG has recently announced that Android 6.0 Marshmallow will be available on their LG G4 headset starting with next week. Initially, it will always be available in Poland, but more countries from Europe, Asia and America will receive the update soon. LG has chosen a phased update program in order to be able to provide the new OS to a small number of devices and receive feedback from the G4 users.
As you may know, Google has recently released the Android 6.0 Custom ROM Images For The Nexus 5, Nexus 6, Nexus 7 (2013), Nexus 9 and Nexus Player, but devices like Nexus 4 or Nexus 10 will not receive Android Marshmallow via the official upgrade channel. But this is not a problem, since the Android source code is open-source. Dmitry Grinberg, one of the XDA developers has released two unofficial Android 6.