T-Mo's "4G" isn't like Sprint's "4G", where Sprint actually runs 3G and 4G networks, using different technologies. If a Sprint enabled 4G phone is in a 4G supported area, its 4G radio can be active and utilize the network.
With T-Mo, any "3G" phone with HSPA+ (aka T-Mo 4G) capable phone will run on the higher speeds in a supported area, using its "3G" radio. Think of it more like 3G and 3G+. Some phones (my Nokia N900) allow you to turn off the 3G radio and use 2G only, so with T-Mo, to turn off "4G", means turning off your 3G radio, and running on 2g (Edge).