The ITU's (International Telecommunications Union) considers two technologies to be officially "4G" LTE advanced and 802.16m WiMax. That means todays flavors of WiMax offered for example by Clearwire don't really qualify. HSPA+ offered by GSM carriers like T-Mobile are not technically 4G although we do agree that if they were compelled to stop calling it 4G then so should Sprint 4G & Clear since the speeds are comparable anyway.
802.16m is expected to receive final approval sometime this year (2011) so we don't expect to see any widespread commercialization until 2012/2013. The big question though is who is still 100% committed to Wimax. Clearwire the first and only major U.S WiMax provider has indicated that they will start testing LTE. Intel the other major backer of WiMax is now also going down the road of LTE ... see intel SMARTi LTE.
802.16m will include 4x2 MIMO in urban microcells on a single 20MHz TDD channel. Theoretical forward link speed of 120Mbps and reverse link of 60Mbps expected (this is of course impractical for any reasonable real-life deployment due to backhaul limitations).