An Android 4.3 build has leaked for the Nexus 4!
Posted just this morning on the XDA-Developers website, the build is currently in TWRP backup format so you'll need to ensure you have the custom recovery installed, push the files to your TWRP backup directory on /sdcard and then restore.
It certainly looks like the real deal, wearing version number JWR66N, as previously seen on the Galaxy S4 GPe 4.3 leak.
With a Google event scheduled for the 24th July, it seems like that 4.3 will be simultaneously released for Nexus and GPe devices on or around that date.
[EDIT: I mirrored the file on MEGA since the Dropbox link is now broken.]