Greepois0n Jailbreak for iOS 4.2.1 is available now, to jailbreak iPad 4.2.1 untethered with GreenPois0n follow the given instructions below.
Update iPad to 4.2.1 (you can skip this step if you already running iOS 4.2.1 on iPad)
Connect your iPad with computer using USB data cable and see this guide to Update iPad to 4.2.1 (come back here to next step after updating iOS 4.2.1)
Jailbreak iPad 4.2.1
Download GreenPois0n and extract .exe file from .zip file
Turn off your iPad, close the iTunes if running
Run the GreenPois0n application and hit the “Prepare to Jailbreak (DFU)” button
Read the on screen instructions to put your iPad in DFU mode
When you successfully put your iPad in DFU mode then hit the “Jailbreak” button
Greepois0n will start Jailbreaking your iPad 4.2.1
Hit the “Quit” button when jailbreak process finished. Wait for the device to load iOS.
Install Cydia on iPad
You will notice a new “Loader” application icon on the iPad Springboard, run this “loader” app
Tap the Cydia icon and then tap the “Install Cydia” button
When Cydia installed reboot your iPad, Run Cydia app.
To remove “loader” app after installing Cydia, tap on Loader app icon then tap “Options” button and “Remove Loader.app” button.