Data speed is determined by coverage, location, device, and other conditions. As long as you have a current plan the data speed will be optimized to the best available at the time.
3G, 4G WiMAX, indicate which data network is used for web access. Each network has different maximum speed capabilities. Both 4G networks are much faster than 3G but neither 4G network is as large as 3G at this time.
Virgin Mobile answered your question in the FAQs:
• Data speeds for 3G devices on the 3G Nationwide Sprint Network range from 600 kbps – 1.4 mbps*.
• Data speeds for 4G devices operating on the Sprint 4G (WiMAX) Network, typical speeds range from 3 – 6 mbps*. The Sprint 4G (WiMAX) Network reaches over 70 markets in the US.
• When in a 4G LTE coverage area, customers may realize average download speeds of 6-8 Mbps, upload speeds of 2-3 Mbps and peak speeds of 25 Mbps*.
*Actual speeds may vary based upon plan or other factors.
• 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, 3G Data Coverage, or Voice Coverage in the boxes above the map.
• 4G LTE devices are currently not available for Broadband2Go.