Phew! As you can see, the 4G landscape is pretty crazy. Another interesting thing to consider: moving from 3G to 4G connectivity is more of a hassle when the two networks' protocols are different. If you drop out of a 4G zone on Sprint, for instance, your device has to switch to a different type of radio band to re-connect to the CDMA/EV-DO 3G network. On T-Mobile's (and presumably AT&T's) HSPA+ networks, this is less of an issue.