GreenPois0n RC5 jailbreaks iphone 4 iOS 4.2.1 untethered (and other iOS devices too).
Unlike Redsn0w 0.9.7, Greenpois0n don’t require you to have SHSH blob of 4.2b3 saved on Cydia. You can jailbreak your iPhone 4 on iOS 4.2.1 using GreenPois0n.
These instructions are to jailbreak iOS 4.2.1 on iPhone 4, iPhone 3Gs, iPod touch 2G/3G/4G, iPad and Apple TV 2G using greenpoison.
Don’t upgrade your iPhone directly from iTunes.
If you’re on Redsn0w tethered jailbreak already, you can run GreenPois0n over it to make your iPhone 4 untethered Jailbreak
UPDATE 4: Windows version of GreenPois0n is now available for download. More details here.
Download (link not working: http://www.greenpois0n.com/ ) GreenPois0n [ Mac OS X Mirror (link not working: http://mirrors.ih8sn0w.com/greenpois0n-osx_rc5_2.zip ) ] Download (link not working: http://www.greenpois0n.com/ ) GreenPois0n for Windows [ Mirror (link not working: http://mirrors.ih8sn0w.com/gp_win_rc5_b2.zip ) ]
Download iOS 4.2.1 for iPhone 4, 3GS, iPod touch 4G / 3G / 2G and iPad
How to Jailbreak iPhone 4 iOS 4.2.1
1. Download GreenPois0n and run it.
2. Connect iPhone 4 to your computer using usb and click “Jailbreak” on GreenPois0n screen.
3. On next page follow onscreen instructions to enter DFU mode.
4. Once your iPhone 4 is in DFU mode, jailbreak procedure will start automatically
5. Once the jailbreak is complete, goto your iPhone 4 homescreen and click loader app and install Cydia.
6. You’re now done, your iPhone 4 jailbreak on iOS 4.2.1 is complete. Now you can install apps using Cydia.