iPhone Dev Team has released the redsn0w 0.9.6 b4 / beta 4 for Windows and Mac OS X to jailbreak iOS 4.2.1 on all the devices. But for some devices ( mentioned below ), the redsn0w 0.9.6 b4 / beta 4 supports only tethered jailbreak. You could get redsn0w 0.9.6 b4 for iOS 4.2.1 jailbreak from the below links.

Redsn0w 0.9.6 b4

In the following devices, redsn0w 0.9.6 b4 supports tethered jailbreak :

  • iPhone 3GS New Bootrom
  • iPhone 4
  • iPod Touch 2G (MC Model)
  • iPod Touch 3G
  • iPod Touch 4G
  • iPad

For the following devices, redsn0w 0.9.6 beta 4 support untethered jailbreak :

  • iPhone 3G
  • iPhone 3GS (Old Bootrom)
  • iPod Touch 2G (Non MC Model)

Download Redsn0w 0.9.6 b4 for Windows from here

Download Redsn0w 0.9.6 b4 for Mac OS X from here