How To Switch Between 3G And 4G On HTC Thunderbolt
If you already have a Verizon HTC Thunderbolt you will most probably notice that there isn't an easy, user-friendly way of switching 4G off or flipping a switch to 3G.
If you're an advanced user who's not afraid to enter test menus and tweak setting you will be able to expose a 3G/4G option which will allow you to choose between the two standards. You have to go into the Dialer and call *#*#4636#*#* which will take you to the Thunderbolt's Test Menu. Head over to Phone Information and Set Preferred Network Type according to your liking: available options are LTE and EV-DO, EV-DO only, and LTE only. To switch back, follow the same steps, only this time choose LTE Mode and select CDMA + LTE / EvDo auto.
Note: Playing with Advanced Setting and Test Menus is dangerous and you are doing it at your own risk!
Source: AndroidCentral Forums