Everybody wants 4g, right now these devices can barely utilize the full theoretical download speeds anyways.... the Vibrant will never actually download 21mbsp, no where near that. These devices can only utilize a specific amount of data, now... that 21mbps can be tethered to a laptop and then yes, that is when you will notice the 4g... regardless its still speedy over 3g, if you really want a T-Mobile 4g stock device, just wait for the next Nexus device, I am sure Google will keep it going especially when they release some major software enabled hardware options. Plus 4g is a hardware thing. No way to update it. If that were the case the carriers couldn't brag about how this phone has 4g and that phone doesn't.
Sent from my Nexus S