I'm not sure if verizon has any sub carriers that are offering 4G?
Also I'm not sure if flashing a phone from Verizon to say Virgin Mobile is possible, like Sprint to Boost Phones.
In the end I'll not recommend trying this either way, Sprint or Verizon. The process involves changing some of the radio info which can easily brick the device if not done perfectly.
give me another minute and I'll track down that Roam control mod.
Here is the original thread for the roaming options. I'm not 100% sure if this still works the newer roms as the flashable zip shown in the thread. "Most" of the newer roms have this baked in, so you more than likely would have an easier time flashing an updated rom with it cooked in.
http://forum.xda-developers.com/showthread.php?t=1707673
Also if you do choose to roam only you may get a nasty gram from sprint. See this thread for more info on excessive roaming on Sprint:
http://androidforums.com/evo-4g-all-things-root/337563-here-goes-nothing-trying-get-vzw.html