The rather hacky method devised by hobbyist Dmitry Grinberg may deter those with little experience in messing with Android. It's not a case of simply flashing the firmware or updating, but by the same token, his method looks relatively straightforward to get up and running. In following the steps outlined at the source, Grinberg claims that users will be able to enjoy all of Android 6.0 Marshmallow's features on the Nexus 4, including GApps and LTE; quite the achievement for what is -- in smartphone terms -- a bit of a relic.
As with any fork of this kind, expect some features to be erratic or not operate to the Android 6.0 specification. Also bear in mind that since the Nexus 4 is coming up for three years old now, even a fully-functional Android Marshmallow installation is likely to be hindered by the aging hardware.
Still, it's good to see that the Android modding community is still thriving, and if you'd like to try this one out for yourself, details can be found at the source.