While the Google Nexus 4 may be a handset that is very good, it is also an old handset and so it won’t be getting anymore updates. However while it won’t be getting updates to Android 6.0 Marshmallow officially, unofficially owners will be able to update as there are ROMs being offered by developers.
There are two Android 6.0 Marshmallow ROMs available for the Google Nexus 6. One comes from Aldrin Holmes and the other is from Dmitry Grinberg. If you have the device and you want to go ahead and install it you can by following the steps below.
• Start by transferring the TWRP file to the device and update/install it.
• Turn off the device and switch it on to Recovery Mode ( Volume up + Volume Down + Power Button).
• Once the TWP UI loads up, choose Wipe > Factory Reset.
• Next Transfers one fo the ROM-GApps into the device and reboot to enter Recovery mode again.
You should now be left holding the Google Nexus 6 with Android 6.0 Marshmallow installed.