4G means 4th Generation....for simplicity's sake, there are 2 standards of 4G: WiMax and LTE
Sprint partnered with Clearwire (an external company who built their own WiMax network) to provide their first 4G connectivity. Sprint doesn't own the WiMax network, they simply have a contract to use it (for a couple more years). All of the other major US carriers have chosen to go with the LTE standard for their 4G networks. Sprint decided to build their own 4G network instead of using Clearwire's...and they also chose to go with LTE. Clearwire's WiMax network will be shut down in several years and LTE is the future. Clearwire's WiMax is not being expanded further so it's footprint is relatively small.
Sprint is aggressively building their own 4G LTE network. Progress on this network's coverage can be found at:
Community Blog List - Sprint 4G Rollout Updates
Sprint's newest phones are all 4G LTE. It looks like they are done introducing new phones that are WiMax capable.
No Sprint phones are capable of both WiMax and LTE.