This little hidden feature was discovered by someone on this forum. The tweak doesn't seem to work on AT&T's network, but it may work on Verizon and T-Mobile. The Verge has more details about this. It boils down to the fact that the antenna on the Nexus 4 is not technically fully LTE-capable, but the tech under it can easily function on certain LTE bands. AnandTech covers a lot of technical detail in their blog post.
You can follow the instructions in the video below to try and force your phone Nexus 4 to work on a 4G network: