iFixit's teardown showed that the device does have a 4G LTE chip, just not 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.
So the Nexus 4 may actually have two unused LTE implementations ?!
Actually, maybe someone can answer this. Does anyone know exactly what the cost is(both in licensing and materials) for a 4G LTE chip vs the antenna?
The company to ask is iSuppli, they specialize in reverse costing. http://www.isuppli.com/Teardowns/Pages/Headlines.aspx