Of all those operating systems Firefox OS is almost useless since it's IMO not stable enough for daily use. Cyanogen Mod will get support from Microsoft. So CM is a "no-go" in the future (you know, it's M$!). Android 6 (not avilable yet) might not run on Nexus 4 anymore. That leaves me only two OSs; Ubuntu Touch and Sailfish OS. I'd prefer the Finnish one but both will do.
There is however one OS which i'd like to see on Nexus 4. That is Debian. There is even a web page dedicated for Debian mobile: https://wiki.debian.org/Mobile . Let's hope my dream comes true one day! Maybe many Nexus 4 users already run Debian. I only need to find those web sites which are telling how to do the trick. Why Debian? It's free software from head to toe.
Firefox OS on Nexus 4 on the right
Cyanogen Mod 12
Android 5 (Lollipop)
Sailfish OS on Nexus 7 wifi (2012) -tablet