From one of the world top iOS jailbreak community, Chronic Dev team has released Greenpois0n jailbreak app which provides the long-awaited untethered jailbreak ability for your 4.21 iOS devices.
Previously, jailbreakers using Pwnage Tool or Redsnow will only have a tethered jailbreak with their devices meaning they have to rerun the jailbreak all over again whenever their iPhone/iPod/iPad turns off or restarts. Not anymore with Greenpois0n which comes in two flavors each for Windows and Mac. I tried the jailbreak myself for my iPad and it runs smoothly using my Windows 7 PC but not with my other Windows XP box. The Mac version of Greenpois0n requires at least Mac OS X 10.6 so no luck for Leopard and older versions. Since Greenpois0n's own website is almost always down, I took the liberty to provide a mirror below. The jailbreak process is fairly simple. Follow the instruction to get into DFU mode and it should be done in less than 5 minutes. After that you can proceed to unlock your phone by installing Ultrasnow in Cydia (does people even buy unlocked iPhones anymore?).
Greenpois0n RC5 for Windows
Greenpois0n RC5 for Mac