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.
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
So the Nexus 4 may actually have two unused LTE implementations ?!