It's only Available for Mac. Windows version will be released soon.
Well now you can Jailbreak 4.2.1 Untethered, Thanks to Greenpois0n by the Chronic Dev.
Download links:
http://mirrors.ih8sn0w.com/greenpois0n-osx_rc5_2.zip
http://cdn.nspwn.com/greenpois0n/greenpois0n-osx_rc5_2.zip
http://greenpois0n.com/ / Their Website is down, so you can open it but Greenpois0n RC4 available nor RC5 so don't bother.
Anyway you might face some issue because their wesbite is down ( overloaded ) and when you try to jailbreak with Greenpois0n you will face some issues too.
Problems and Solution:
* Problem 1 : I got stuck in "Greenpois0n initialized", retried, re updated. It remains the same.
* Solution : open "terminal", type SUDO, open Greenpois0n as "Show package contents", go MacOS folder & drag greenpois0n to terminal & hit enter and check the logs in terminal.
* Problem 2 : My iPhone / iPod is showing blank (black) screen after jailbreak, iPhone / iPod is still in DFU mode after applying jailbreak.
* Solution : Leave your iDevice for two minutes, now press and hold both power and home button at the same time for 10-15 seconds until your iPhone / iPod boot as normal.
* Problem 3 : Greenpois0n loader is crashing when I install Cydia, I am unable to install cydia via greenpois0n loader.
* Solution : For those having trouble getting Cydia via the GP Loader, just let redsn0w install it (don’t choose the Monte option)
For The First problem, you can retry couple of times it will work without trying this solution Because the Jailbreak gets downloaded from their server. Sadly, their servers are down
For last Problem, you need to use Redsn0w to install Cydia (just tick Install Cydia in Redsn0w). That's what i just did.
Oh Jailbreakin with Greenpois0n is very easy, just follow the instructions to get to DFU mode in the application.
UNLOCKERS STAY AWAY FROM IT.