Android 4.2.2 update blocks unofficial LTE support on Nexus 4
"The latest Android update includes both software changes and a new baseband software image for the Nexus 4 that have effectively disabled LTE on the device."
Not sure why it as blocked, but likely due to no FCC certification for LTE. If so it should remain officially blocked in the future.
Might be worth holding off on the update if you're on one of the supported networks and want to play around with LTE. XDA might be able to bring that feature back, but maybe not once the baseband is updated.