I just upgraded to a 4G modem (Novatel USB551L), and find myself trying for up to an hour on most days to finally get a connection. More specifically, the modem connects, but only sends and never receives, making the connection useless, because nothing will load. I disconnect/connect for 20 minutes to an hour, before I finally have success. This never happened to me when I had the 3G modem.
Did you ever get an answer to this question? I, too, am having the same problem with my USB551L and so far, Verizon's phone support hasn't been able to help me. I never had this issue w/my 3G aircard.