Not long after iOS 4.2.1 was released, Dev team has brought some updates related to the newest firmware. They announced that the release of Redsn0w 0.9.6b4 (which is based on GeoHot’s limera1n bootrom exploit exploit) is able to jailbreak the iOS 4.2.1 on iPhone, iPad and iPod touch.
What differs Redsn0w 0.9.6B4 from Redsn0w 0.9.6B3 I hear you ask? Because there is no difference between the final version of iOS 4.2.1 and the iOS 4.2.1 GM, the Redsn0w 0.9.6B3 can theoretically be applied on the iOS 4.2.1. However, the Dev team has recompiled the jailbreak tool so that it works perfectly with iOS 4.2.1 IPSW.
The redsn0w version 0.9.6b4 is now ready for your 4.2.1 jailbreaking pleasure. Don’t forget to read Dev team full announcement.