Redsn0w0.9.6b4 will jailbreak all devices except iPhone and iPod Touch 1G on iOS 4.2.1
Old-Bootrom iPhone 3GS
Non-MC iPod Touch 2G
New-Bootrom iPhone 3GS
MC iPod Touch 2G
iPod Touch 3G
iPod Touch 4
Download iOS 4.2.1 firmwares:
Mac - http://www.iphone-storage.de/redsn0w_mac_0.9.6b4.zip
Windows - http://cdn.nspwn.com/redsn0w_win_0.9.6b4.zip
! Unexpected Error !
This is an error that I received on my Macbook late 2006 model. Redsn0w0.9.6b4 skipped DFU mode and gave me an unexpected error during the jailbreak process. I did a bit of research and found no real answer other than to try another computer. As you can see, thats what I did and it worked using a 27" iMac. You have to remember that this is still in Beta...