RedSn0w was recently updated to jailbreak iOS 4.2.1. While this is excellent news, it doesn’t come without its drawbacks as only the iPhone 3G, older iPhone 3GS, and non-MC iPod touch 2G get the untethered jailbreak.

Other devices get a tethered jailbreak, which means that every time you reboot your iPhone, you will have to plug your iPhone in your computer, and launch RedSn0w to start your iPhone. Hopefully the Dev Team will come up with a way to bypass this soon.

If you are an unlocker, you should not update to 4.2.1 yet. You will lose your chances of unlocking. Wait for PwnageTool, which should be updated relatively soon.

This step by step guide will show you how to jailbreak your iPhone iOS 4.2.1 with RedSn0w. This tutorial was made using an iPhone 3GS but the steps are the same, no matter what device you are using…

Step 1: Make sure iTunes is updated to its latest version. Make sure your iPhone is on 4.2.1. If not, update it via iTunes.

Step 2: Download RedSn0w 0.9.6b4 and the firmware for your device from our downloads section.

Step 3: Launch RedSn0w and browse for the firmware you downloaded in step 2.

Step 4: Select to Install Cydia.

Step 5: Make sure your iPhone is turned off and plugged in your computer. If it’s not, plug it in, then turn it off. Click next, and be ready to get your iPhone in DFU mode.

Step 6: Hold the Home and Power button together for 10 seconds. Release the Power button while still holding the Home button for a few seconds.

Step 7: Your iPhone will run lines of codes. When it’s done. Click Finish. You’re done!

Now if you have a newer device such as the iPhone 4, every time you reboot your iPhone, you will have to launch RedSn0w and choose the “Just boot tethered right now” option to boot your iPhone.

There is currently no unlock for iOS 4.2.1 but we will keep you updated as soon as it is released.

Any question, feel free to ask below.