The Overdrive Pro must be in an area covered by Virgin Mobile's 4G WiMAX network to operate in 4G mode. And it can't access the 4G LTE network.
Yes, you can set the preferences to only use 4G WiMAX. See the Overdrive Pro user manual at http://www.virginmobileusa.com/resources/phones/prepaid/manual/OD-Pro-user-guide.pdf
See Mobile Broadband Information and Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
• Check your COVERAGE on the top line of this webpage.
Follow the link at this FAQ: http://virginmobileusa.custhelp.com/app/answers/detail/a_id/3164/kw/coverage
Be sure to select your coverage interest such as 4G WiMAX or data coverage(3G) in the boxes above the map.
'Coverage is not available everywhere and varies based on a number of factors. Our coverage maps provide high level estimates of our coverage areas when using your device outdoors under optimal conditions. Coverage isn't available everywhere. Estimating wireless coverage and signal strength is not an exact science. There are gaps in coverage within our estimated coverage areas that, along with other factors both within and beyond our control (network problems, software, signal strength, your wireless device, structures, buildings, weather, geography, topography, etc.), will result in dropped and blocked connections, slower data speeds, or otherwise impact the quality of services...'