ChronicDev Team has just released the latest version of greenpois0n, which is said to work for iOS 4.2.1. Unlike previous releases, this version is UNTETHERED, which basically means you wouldn’t have to hook it up your PC or Mac everytime you shut off the device.
The download is available at http://greenpois0n.com/ but at the time of writing, the website is still down due to heavy traffic. You could also try downloading from this mirror.
Just be sure to backup all your files and restore your device before trying to jailbreak. Let us know in the comments section if it works out for you.