The Chronic Dev Team has released its GreenPois0n tool for Windows and Mac to untether jailbreak iOS 4.2.1. Unlock for iPhone 4 running iOS 4.2.1 is not available yet so if you have to unlock your device then stay away for now. iPhone 3GS users will have to use a custom ipsw to upgrade to iOS 4.2.1 using Pwnage tool if they aren’t already using iOS 4.2.1 If you are using iOS 4.2.1 on your iPhone 3GS then jailbreak with GreenPois0n using this guide then follow this guide (link) to unlock with ultrasn0w.
What we need
iOS 4.2.1 for your device
- iOS 4.2.1 for iPhone 3GS
- iOS 4.2.1 for iPhone 4
Note: If you are already running iOS 4.2.1 with tethered jailbreak on your device then leave step 1 and skip to step 2.
1 ) If your device is not running iOS 4.2.1 then update it using iTunes (iPhone 4 unlockers should stay away)
2 ) Extract the GreenPois0n file to your desktop
3 ) Now open the GreenPois0n.exe file you just extracted on your desktop.
4 ) Turn off your device and click on Prepare to DFU button
5 ) Now Carefully follow the instructions on your screen to put your device in DFU mode
6 ) Now leave your device and computer for sometime while GreenPois0n prepares to jailbreak your device. You should see some programming codes and GreenPois0n image on your device (It would take a few minutes).
7 ) When prompted click on the Quit button.
8 ) Now when your iPhone restarts you will see a new icon on your Springboard named ‘Loader’. Tap on this icon and Press to select Cydia then press the large Install Cydia button that appears on your device.
9 ) Once Cydia has been installed on your device successfully you can remove ‘Loader’ by pressing the Options button then the large red Remove Loader.app button.
10 ) Restart your iPhone to run Cydia.
Now your device is jailbroken, you can unlock it (if possible) using Ultrasn0w, for help follow this guide (Link).