NOTE: All the standard warnings mentioned previously still applies.
Download iOS 4.2.1 for iPhone 4, 3GS, 3G, iPad, iPod touch
Download Redsn0w 0.9.7b4 for Mac
Download iTunes 10.1.1 for Mac
Now the public version Redsn0w 0.9.7b4 for Mac has finally come out. The next version Redsn0w 0.9.7b5 will be testing on windows. Windows users are closer to the Untethered Jailbreak iOS 4.2.1 iPhone 4, iPad, iPod Touch 4.
The differences between the Mac and windows version Redsn0w 0.9.7 may not be many. But before the Redsn0w 0.9.7b5 for Windows come out, please don't try untether jailbreak your iOS 4.2.1 iPhone 4, iPad, iPod touch. Cause there may be some incompatible that will lead to the crash problem of your devices.