The instructions below works to jailbreak iPad 4.2.1 with Greenpois0n RC5, so continue reading to know the complete process below.
Warning: Users who rely on Ultrasn0w unlock must update to stock 4.2.1, otherwise stay away from Greenpois0n RC5 tool as they will lose the unlock.
Note: This guide is written for Mac OS X users.
Jailbreak iPad 4.2.1 with Greenpois0n RC5
Create a folder on your desktop, download Greenpois0n RC5, and unzip it in your created folder, suppose with the name Untethered Jailbreak
Double click on Greenpois0n icon to launch it inside your folder
Attach your iPad to Mac, power it OFF, click on Jailbreak button on Greenpois0n screen
Greenpois0n will instruct to put your iPad into DFU Mode, so continue with those instructions carefully
When your iPad will enter into DFU mode successfully, the jailbreak will get start and on completion of process just click Complete! button
After restart of your iPad you will find Greenpois0n Loader app sitting on Springboard, so tap on it to launch that
Select Cydia and tap on Install Cydia button
Once Cydia will be installed, it will prompt to remove Loader from your iPad, so tap on Remove button to do that
Reboot your iPad by pressing Home button
Enjoy, you have jailbroken iPhone 4/3GS over iOS 4.2.1 with Greenpois0n RC5.