OK. After having no problem switching back and forth, I now cannot switch back to 4G either!
Try wiping data on the HTC LTE app. Factory reset is the next option. If those don't work you will probably have to re flash software. I lost 4G radio as well; had to reflash.
Thanks, the widget just sends you to that menu so the widget and *#*#4636#*#* should be the same. When I choose LTE/CDMA/EvDo the 3G/4G LTE indicator disappears and the signal strength indicator stays gray. I can always go back to CDMA auto (PRL) and get 3G, but nothing gets 4G LTE to enable. First times I tried this it worked fine. I switched back and forth multiple times. 4G is spotty at home so keeping on 3G makes the data more reliable. Switching was trivial in GB, not sure why with Verizon's ICS builds it has been removed from "standard" settings...
OK. Got it switching again! I had enabled Data Roaming and with that enabled I was unable to switch back and forth between 3G only and 4G. With that disabled, all is fine again.