The Dev-Team Blog was just updated with a new redsn0w version 0.9.6b4 jailbreak (based on Geohot's Limera1n bootrom exploit) for iOS 4.2.1. Unfortunately, iOS device owners won't find jailbreaking as easy as navigating to a website. And the 4.2.1 jailbreak is currently "tethered" so owners of the iPhone 4, iPad, and newer iPhone 3GS and iPod touch will have to reattach to a PC or Mac to boot into a jailbroken state each time your device loses power or needs a reboot. Those of you who rely on the ultrasn0w unlock should hold off for now -- good advice for anyone not comfortable with terms like "SHSH blobs" or "Cydia."
In the following devices, redsn0w 0.9.6 b4 supports tethered jailbreak :
iPhone 3GS New Bootrom
iPod Touch 2G (MC Model)
iPod Touch 3G
iPod Touch 4G
NOTE: Those of you who rely on a carrier unlock MUST STAY AWAY from Redsn0w and iOS 4.2.1.
Follow the step by step instructions posted below to jailbreak iOS 4.2.1 with Redsn0w 0.9.6b4:
Step 1: Download iOS 4.2.1 for your version of iOS device.
Step 2: Update/Restore to iOS 4.2.1 firmware using the IPSW file you just downloaded via iTunes 10.1.
Step 3: Start Redsn0w 0.9.6b4 and point it to iOS 4.2 to proceed with the jailbreak.
Step 4: Now select “Install Cydia” and click “Next”. Optionally (if available), you can also select any other option you like.
Step 5: Now make sure your device is both OFF and PLUGGED IN to the computer before you click “Next”.
At this stage you will be presented with a screen where you will have to quickly hold “Power” button and then while holding the “Power” button, you will now have to hold “Home” button too. Now after few seconds release “Power” button but keep holding “Home” button until installation begins.
Step 6: Sit back and enjoy while redsn0w will do all the rest for you. Once done, your iPhone or iPod touch will restart automatically and you will find it jailbroken on the latest iOS 4.2.1 firmware.For iPhone 4, 3GS (New Bootrom), iPod touch 4G / 3G / 2G, iPad ONLY:
Step 7: Now from here onwards, whenever you want to SSH into your device, you must first run your device in the so called “jailbroken state” on every reboot by using “Just boot tethered right now” option in Redsn0w.