Today windows version of greenpois0n is released so it is possible for everyone to untether their devices. Even if they were jailbroken previously with redsn0w. If you have used Monte to jailbreak you must restore to clean 4.2.1 firmware.
Steps are quite easy.
1. Enter DFU mode.
2. Run Greenpois0n and press jailbreak.
Download Greenpois0n from the official site or from one of my mirrors:
Linux Version - Still Unavailable