Sprint has toyed with the idea of adopting LTE to supplement its WiMAX services, as both it and Clearwire would together have enough spectrum to support both technologies if they so desired. The companies will also likely roll out the 802.16m ("WiMAX 2") standard at some point, although that might not occur until 2013 since device certification for the standard is slated to start this year and it will take some time for carriers to commercially deploy the new technology over their existing networks.
Conclusion: Buying "4G services" from a carrier really doesn't tell you anything about its coverage or overall quality. Verizon's LTE network currently offers the fastest data rates of all the networks, although it isn't yet available throughout the entire country and its 3G backup network is significantly slower. T-Mobile and AT&T are currently getting good mileage with their HSPA+ networks and they should remain competitive with Verizon as they start rolling out LTE over the next two years. Sprint's WiMAX network also gets good speeds, although there remains a good deal of uncertainty about the future of its partners at Clearwire.
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