If you have an unlimited 3G mobile broadband account, you can't upgrade it at all to 4G. The only way to still get unlimited on 4G is with a grandfathered unlimited phone plan. And you have to pay full price on any devices, and can't use the upgrades in almost every case I've seen. I would dump the 3G account and look for someone selling their phone plan with unlimited data and do an Assumption of Liability and take over their plan.
Thank you for the suggestion of taking an AoL on a phone plan.
I am just going to dump Verizon. I had been considering it for a while as their 3g service has steadily declined in quality over the past two years, and any time I called in about it support was limited to "have you tried reinstalling VZAM?"...sigh. I already have a high end Sprint plan w/hotspot I've been using when my Verizon service has been borderline unusable. While their coverage is lacking compared to Verizon, their service has been reliable and doesn't drop to 6kb during primetime, as my Verizon stick often would.