If you have the Verizon HTC Thunderbolt, you may notice that your battery life goes down a bit too fast. This is because the 4G antenna takes up a lot of power. There is no easy way to switch it between 3G and 4G, but there is a way.
To switch the phone between 3G and 4G, go into the Dialer and call *#*#4636#*#*
This will bring you to Thunderbolt’s Test Menu. Go to Phone Information and Set Preferred Network Type however you want. Options are: LTE and EV-DO, EV-DO only, and LTE mode. Remember, this is a Test Menu so be careful what you change!
This tip was found on http://pocketnow.com/android/how-to-switch-between-3g-and-4g-on-htc-thunderbolt. They also have a screenshot on what you can expect to see.