pastert33 is correct. the original Sprint 4g network was WiMAX technology. The new 4G is LTE. Basically different radio frequencies. So the radios, being locked to a single frequency band, are incompatible with one another.
Most Gnex users, such as myself, will simply disable the 4G radio and use the GNex as a 3G device until LTE comes to our area. Which for me should be just about the time the next Nexus phone comes out. *shrug* What are ya gonna do? My wife's iPhone doesn't even HAVE a 4G radio. So I can't complain.