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

Step 1

Create a folder on your desktop, download Greenpois0n RC5, and unzip it in your created folder, suppose with the name Untethered Jailbreak

Step 2

Double click on Greenpois0n icon to launch it inside your folder

Step 3

Attach your iPad to Mac, power it OFF, click on Jailbreak button on Greenpois0n screen

Step 4

Greenpois0n will instruct to put your iPad into DFU Mode, so continue with those instructions carefully

Step 5

When your iPad will enter into DFU mode successfully, the jailbreak will get start and on completion of process just click Complete! button

Step 6

After restart of your iPad you will find Greenpois0n Loader app sitting on Springboard, so tap on it to launch that

Step 7

Select Cydia and tap on Install Cydia button

Step 8

Once Cydia will be installed, it will prompt to remove Loader from your iPad, so tap on Remove button to do that

Step 9

Reboot your  iPad by pressing Home button

Enjoy, you have jailbroken iPhone 4/3GS over iOS 4.2.1 with Greenpois0n RC5.