I really wish there was a simple 4G LTE toggle widget for the Thunderbolt. There are a couple of apps in the Market which open the testing menu; however, it should be easier.
So, why isn't there one? Looks like there is not a predefined network connection type in the API: http://developer.android.com/reference/android/net/ConnectivityManager.html.
Normally I'd expect to see a ConnectivityManager.TYPE_LTE but that type doesn't exist yet. The signature for setNetworkPreference is int, so I may be be able to call getNetworkPreference to determine the LTE integer value, store it in a constant, and then reference it later. If the signature is really the enum, I may be out of luck.
Maybe I'll try this when I get home - if I have time. Maybe someone else has already tried it...