How to Jailbreak a 4.2.1 firmware iPod Touch, iPhone, or iPad with Redsn0w 0.9.6b4
If you've downloaded the latest iOS version, 4.2.1, and haven't jailbroken your device yet, you've come to the right video. It will teach you how to use Redsn0w to jailbreak your device and install new firmware from here. Now you can whatever you want with your iDevice.
Caution: there is no unlock for the new firmware yet, so if you need an unlock don't update your device or be patient.