Why would they put in the chip if it didn't have an antenna?
I think T-Mobile is selling their HSPA+ as "4G", even though it's not using LTE yet. So it's not that no GSM carriers would advertise the phone as "4G".
Perhaps they designed and/or manufactured the circuit boards before they had the frequencies all sorted out. Perhaps they need the chip in there for patent licensing reasons.
The real reason is that they went with the Qualcomm S4 SoC, which has an LTE modem on-die. It's likely that it would be too expensive to have Qualcomm manufacturer an LTE-less chip for one device.
According to http://www.qualcomm.com/chipsets/snapdragon the Qualcomm Snapdragon APQ8064 has LTE "on select processors". But it's still a separate chip from from the WTR1605L LTE chip.
So the Nexus 4 may actually have two unused LTE implementations ?!
Now this boggles the mind. This could be explained by the Nexus 4 sharing the internals of the Optimus G, but why opt for the off-die LTE modem in the first place?
http://www.ifixit.com/Teardown/Nexus+4+Teardown/11781/3