Greenpois0n For 4.2.1

 
This video will teach you how to Jailbreak iOS 4.2.1:

Devices it works for:
iPhone 4 (For AT&T AND Verizon Models)
iPhone 3GS
iPod Touch 2nd Gen (MC and Non-MC Models)
iPod Touch 3rd Gen
iPod Touch 4th Gen
Steps:
1. Download Greenpois0n for your OS (Windows and Mac) – http://www.greenpois0n.com
2. Open Greenpois0n + iTunes and set your iDevice into DFU mode by following these instrucations:
a. Hold down the home and power button for a total of 10 seconds
b. Let go of the power button and continue to hold the home button for 20 seconds
3a. Hit “Jailbeak” in Greenpois0n – If your on a Mac
3b. Hit “Prepare to Jailbreak (DFU)” in Greenpois0n then “Jailbreak!” – If your on a Windows
—Troubleshooting Steps—
4. (conditional) If your iDevice just goes to the Apple logo (instantly) or if it just says failed then follow steps 2-3 again!
5. (conditional) If step 4 (conditional) doesn’t work then you MUST restore to 4.2.1
6. (conditional) If you don’t have “Loader” to install Cydia AND IT WORKED ALL OF THE WAY then run Redsn0w over it – here is a tutorial: http://www.youtube.com/watch?v=H-h597PR_KI
**NOTE** You must have the latest version of iTunes!
If you have any other questions be sure to visit: http://www.jailbreakqa.com – the developers also answer questions there

Here is ANOTHER video (this time it’s Greenpois0n running over Redsn0w):

If you have any other questions be sure to visit: http://www.jailbreakqa.com – the developers also answer questions there