Most comprehensive 4G availability (so far) - Intro
As it stands on May 2011, the only carriers with a genuine 4G network are Verizon and Sprint. Verizon is rolling out LTE, or Long-Term Evolution, a genuine fourth-generation network created by a consortium known as 3GPP, with theoretical peak data rates of 300Mbps downloads and 75 Mbps uploads.
Sprint uses WiMAX, which is promoted by ClearWire and developed by a consortium led by Intel. It has down/up speeds of 75Mbps/30Mbps, way better than 3G but falling short of LTE. The thing is ClearWire has recently hinted that it will migrate to LTE in the future, so that leaves a huge question mark as to where Sprint is going vis-a-vie WiMAX or LTE.
T-Mobile is hyping "the largest 4G network," but it's not true 4G. It's network is HSPA+, an amped-up 3G technology, albeit very amped up. In theory, it can max at 84Mbps down and 22Mbps up but test of T-Mobile hotspots show it tops out in the 20Mpbs range. Hardly anything to complain about, but it's still not genuine 4G.
Now, AT&T has done some HSPA+ deployments of its own, so T-Mobile's network won't be orphaned if this deal goes through. But by and large, AT&T is now moving forward with LTE, not HSPA+.