I've been trying to find the 4g on/off settings myself and it's been a real mystery.
However I did find these settings
Settings>more settings>mobile networks>access point names>
There are two choices from here
EHRPD - Verizon Internet
LTE - Verizon Internet
I assume that the EHRPD setting is 3g and the LTE is, well...LTE 4G.
However, I haven't had time to experiment so your mileage may vary.
Isn't this an ICS OS issue? I remember the RAZR MAXX(w/ICS) and DINC 4G LTE are lacking the 3G/4G toggle menu.
Experiment...u are correct. Although...someone found a programming procedure to fix that on the razr maxx. Hope someone does for the gs3 too.