Chronic Dev Team has just released greenpois0n to jailbreak iOS 4.2.1 for Windows for iPhone 4, iPhone 3GS, iPod Touch 4G,iPod Touch 3G and iPad.

In this guide, we will take you through the step by step procedure to jailbreak your iPhone 4 or iPhone 3GS on iOS 4.2.1 using greenpois0n.

Here are some important points to note before you proceed:

  • Please note that jailbreaking your iPhone may void your warranty and hence proceed with caution and at your own risk.
  • This guide is meant foriPhone 4, iPhone 3GS users only. iPod Touch 4G and iPod Touch 3G, iPod Touch 2G users can checkout this step-by-step guide and iPad users can checkout this step-by-step guide.
  • Greenpois0n supports iOS 4.2.1 and iOS 4.1. So you need to first update your iPhone with iOS 4.1 or iOS 4.2.1 to use greenpois0n. Please do NOT upgrade to iOS 4.1 or iOS 4.2.1 if you want to unlock your iPhone.
  • Please note that this guide is only meant for Windows users. Mac users can refer to this step-by-step guide.
  • greenpois0n does NOT hactivate your iPhone, so your iPhone should be activated via iTunes first.
  • Do not forget to backup your iPhone before you proceed.
  • Please ensure you are running latest version of iTunes, iTunes 10.1 or later.

You can follow these step-by-step instructions to jailbreak your iPhone:

  1. Go here to download the latest version of greenpois0n.
  2. Double click the greenpois0n zip file to extract the file and double click greenpois0n to launch the application.
  3. Next connect your iPhone to the computer and power it off.
  4. Then click on the Prepare to Jailbreak (DFU) button:
  5. You will be prompted to put your iPhone in DFU mode. Greenpois0n will take you through the necessary steps.
  6. Press and hold the Sleep/Wake button for 2 seconds.
  7. Keep holding the Sleep/Wake button and also simultaneously also press and hold the Home button for 15 seconds.
  8. Then release the Sleep/Wake button but continue holding theHome button for 15 seconds.
  9. Once the iPhone enters DFU mode, you should see Ready to Jailbreak, now click on the Jailbreak button.
  10. You should see greenpois0n jailbreaking your iPhone.
  11. After jailbreaking is complete, you should see the Quit button, click Quit button to close the application. You should see text in verbose mode scrolling on your iPhone’s screen.
  12. Your iPhone will reboot. After it has rebooted, you’ll notice that there is a new Loader application on your iPhone’s home screen.
  13. Tap on it to launch it. Select Cydia and then tap on Install Cydia button.
  14. After you have successfully installed Cydia, you can uninstall the Loader application as it is no longer required. Tap on theOptions button and then Remove button to uninstall it.
  15. That’s it, your iPhone is successfully jailbroken and you should see Cydia on the homescreen. Reboot your iPhone and launch Cydia to install the latest updates.

Troubleshooting steps:

  • If you don’t see Cydia in the Loader app after launching it then you can install Cydia using Redsn0w 0.9.6 RC8 (download link). You should just select Install Cydia from the list of options and don’t use the Monte option.
  • Greenpois0n recommends a clean restore so if the above steps don’t work then you can try to restore your iPhone if you desperately want a jailbreak as it is not ideal.
  • Connect your iPhone directly to your computer (not via a USB hub).

Updated: Greenpois0n does not support iPhone 3G, we’ve updated the post to remove references to iPhone 3G. Apologies for the inconvenience caused.

You can follow on Twitter or join our Facebook page to keep yourself updated on all the latest from Apple and the Web.