Apple released iOS 4.2.1 and iPhone dev team released Redsn0w 0.9.4b4 jailbreak for iPad (tethered). You can follow our guide to Jailbreak iOS 4.2.1 on iPad with Redsn0w. We got some reports that after installing mobile-substrate their iPad stuck on Apple boot logo reboot loop.
Folks over iHackintosh also faced the same issue and they fixed it with redsn0w “I jailbroke my iPad iOS 4.2.1 (tethered) and updated mobile substrate, Cydia asked me to reboot my iPad and i did. iPad stuck on Apple logo for 3-4 minutes then turned off and booted again but again didn’t surpassed the Apple logo for 3-4 minutes and cycle continues. I tried hard reboot but no luck. Then i tried “just boot tethered now” and it works like charm.
3. Follow the on screen instructions to enter in to DFU mode and your device will be boot properly in seconds.