Settings > Wireless & networks > More > Mobile Networks > Network Mode
Mines on 3 UK, but it's unbranded and unlocked.
3 have said that they will send out an ota update to enable 4G on their handsets.
There are a few cities already enabled, so if you can get somewhere that's already on, you might be able to test your handset.
Sent from my C6603 using xda app-developers app
HTC Hero > Motorola Defy > SE Xperia Arc S > HTC Sensation > Xperia S > Xperia Z + Nexus 7