Chronic Dev Team released the Greenpois0n, Jailbreak for iOS 4.2.1 Untethered and its is available to download as link mentioned below. You can use Greenpois0n to jailbreak iPhone 4, 3GS, iPod Touch 4G, 3G, 2G and Apple TV 2.
Greenpois0n is available for Windows and Mac OS X
Greenpois0n for Linux [Not Available Yet]
- How to jailbreak iPhone 4 4.2.1 untethered using Greenpois0n
- Jailbreak iPad 4.2.1 Untethered using GreenPois0n
- How to Update iPhone 4 to 4.2.1 without updating baseband