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The iPhone Dev team has updated Redsn0w to jailbreak the latest iOS 4.2.1 firmware for all iDevices. Before you continue you must know that for the newer gen devices such as 3GS new bootrom, etc, etc, you will have a semi untethered jailbreak.

A semi untethered jailbreak means if your iphone goes dead or if you install a cydia tweak that requires you to reboot you will need a computer to be able to launch jailbreak apps. This isn’t completely tethered like people think. Semi untethered allows you to reboot without a computer fine but you won’t be able to use jailbreak apps as they will keep crashing until you reboot with a computer and run redsn0w to boot up.
How To Jailbreak iOS 4.2.1:

Warning: Do Not Use This Method If Using An Unlock or You Will Lose It.
  • Download Redsn0w .9.6b4 Windows (Here), Mac (Here)
  • Download iOS 4.2.1 for your device from HERE
  1. Open up redsn0w, choose browse, and find the firmware you just downloaded.
  2. After its identified choose next
  3. You will see a lot of options to choose from, choose install cydia then click next
  4. Turn your iDevice off and then choose next, follow the onscreen instructions to put your device in dfu mode.
  5. Enjoy your jailbroken iphone but wait if you a a newer device another step
  6. For newer iDevices you must boot up using redsn0w’s “just boot tethered feature”
  7. Now you can enjoy you jailbroken device.