Really? Exactly what 4G technology do they currently use?
Yeah, about that.... http://newsroom.clearwire.com/phoenix.zhtml?c=214419&p=irol-newsArticle&ID=1456462&
T-Mobile has already announced plans to support LTE (4G). They will utilize the same technology as AT&T and Verizon. Sprint decided to go with WiMax. This makes them unique in the US and makes it difficult for them to 'convince' cell phone makers (like Apple) to build a specific version of a product for just Sprint.
Even the CEO of Sprint is leaving the door open to LTE as he recently stated that there is a chance that they might have to support this in the very near future. Another parallel network (Sprint, Nextel, WiMAx, and now LTE) and another mess.