Canonical released a public version of the Ubuntu Touch developer preview earlier today. Getting this operating system up and running on the Nexus 4 is both straightforward and rather simple. If you have experience flashing custom ROMs and other mods to your Android devices, this process should give you no problems.
Files you will need:
TWRP for the Nexus 4 — http://techerrata.com/browse/twrp2/mako