The jailbreak for iOS 4.2.1 is now available via Redsn0w 0.9.4b4 by iPhone Dev Team. Here is the full step by step guide and details for jailbreaking iOS devices running iOS 4.2.1

Before you go for the guide, you must know:

- This jailbreak is for iPhone 3G, iPhone 3GS, iPhone 4, iPod Touch 2G, iPod Touch 3G, iPod Touch 4G and iPad running on iOS 4.2.1 only

- Except for iPhone 3G, iPhone 3GS old iBoot and iPod Touch 2G MB model, this jailbreak is tethered. That means every time you reboot your device, you will have to jailbreak it again.

- This will not unlock your iPhone. If your iPhone is not factory unlocked, don’t try this jailbreak because no unlock for iOS 4.2.1 is available yet.
Update 1: Unlock for iPhone 3G and 3GS on iOS 4.2.1 with baseband 04.26.08, 05.11.07, 05.12.01, 05.13.04 and 06.15.00 is available, click here for complete guide and details. Whereas the unlock for iPhone 4 on iOS 4.2.1 is not available yet so don’t go for this jailbreak if your iPhone 4 relies on software unlock.

- I will not be responsible for any loss of your device while attempting this jailbreak, do it on your own risk

How to jailbreak

1. Download and install the latest version of iTunes
2. Connect your device to iTunes 10.1 and update your device to the downloaded iOS file. To do this, in iTunes under devices, select your device and in ‘summary’ tab under the ‘version’ click ‘update’ while pressing Shift key. File browser will open and select the downloaded iOS file follow the instructions.
3. Download and run Redsn0w 0.9.6b4 for Windows (click here for Mac version)

   

4. Browse for the iOS 4.2.1 IPSW and click Next. You can download the required version of iOS from the following links:

   - iOS 4.2.1 ipsw for iPhone 4.

   - iOS 4.2.1 ipsw for iPhone 3GS.

   - iOS 4.2.1 ipsw  for iPhone 3G.

   - iOS 4.2.1 ipsw for iPod Touch 4G.

   - iOS 4.2.1 ipsw for iPod Touch 3G.

   - iOS 4.2.1 ipsw for iPod Touch 2G.

    

5. Check ‘Install Cydia’ and click Next.
   
6. Turn off your device and plug it in to the computer and click Next.
   
7. Connect your device in recovery mode. To enter recovery mode, Press and hold the “Home” button and the “Sleep/Wake” button at the same time. When As the screen goes black release the “Sleep/Wake” button.
8. There will be a screen where you have to quickly hold the “Sleep/Wake” button and just after that also hold the “home” button. After few seconds release the “Sleep/Wake” button but keep holding “Home” button until redsn0w starts the installation.
9. After installation, your device will restart to iOS 4.2.1. Done.