Recent testing indicates that AT&T may be capping speeds of its 4G devices (see article: http://www.pcmag.com/article2/0,2817,2381578,00.asp)
With the standards for 4G being ill-defined at best, carriers can legally get away with marketing 4G devices that are actually slower than 3G on the same network. http://www.pcmag.com/article2/0,2817,2381915,00.asp
So buyer, beware! That new mobile phone or tablet might not be as zippy as the advertisers would have you believe.