The redsn0w 0.9.6 b4 could be used to jailbreak iPad 4.2.1 iOS. If you want to jailbreak iPad on iOS 4.2.1 firmware, then here are the steps to do it using redsn0w 0.9.6 beta 4. But remember, this is a tethered jailbreak. If you want to SSH into your device, you need to put your device on a jailbroken state on every reboot.

Jailbreak iPad iOS 4.2.1

1. Download iOS 4.2.1 for iPad from here.

2. Update your iPad to iOS 4.2.1 using iTunes.

3. Download Redsn0w 0.9.6 b4 from here.

Now follow the rest of the steps given here to jailbreak iPad on iOS 4.2.1.