Currently greenpois0n RC 5 for 4.2.1 firmware is now available for both Windows and Mac. While not required it is recommended that you Restore to iOS 4.2.1 before attempting the jailbreak.
It supports almost all devices on iOS 4.2 including:
- iPod touch 2G (all bootroms)
- iPod touch 3G
- iPod touch 4G
- iPhone 3GS (all bootroms)
- iPhone 4
- Apple TV 2
Click here for our full tutorial.