NTT DoCoMo announced results from a recent test of 4G. It looks even better than first thought (promised to be links of up to 100 megabits per second):
The company said that the test achieved a maximum downstream data rate of 300Mbps with an average rate of 135Mbps in a car running at the speed of 30 kilometers per hour in areas 800m to 1km away from the 4G wireless base stations.
Faster speeds were attained by using multiple input, multiple output (or MIMO). However this means using multiple antennas, something mobile devices might not have the luxury of doing (at least with today's tech).
The definition of 4G that the International Telecommunication Union - Radiocommunication Standardization Sector (ITU-R) approved in June 2003 states that the data rate should be around 100Mbps when moving fast like in a train, and 1Gbps when not moving.