Bundles is an unofficial release by iPhone hackers Johnny Franks.You can use this bundle with existing version of Pwnagetool to create custom firmware
file which is jailbroken on the newly released iOS
If you cant wait for the official tools, you can give this semi-untethered jailbreak method a go! With semi-untethered jailbreak, you will have to reboot your device into the so called “jailbroken state” every time you want to run Cydia.
The complete detailed step by step guide on how to jailbreak iPod touch 4 on iOS 4.2.1 with PwnageTool, followed by instructions on how to patch Cydia on iOS 4.2.1 can be found here.
Download iOS 4.2.1 PwnageTool Bundle for iPod touch 4G