I **believe** what you want to do is;
Get the latest TinyUmbrella. Get the 4.2.1 ipsw file for the iPad. Note that you may need to rename it from *.zip to *.ipsw if you are using windows...
Open TinyUmbrella
Plug in your iPad and save SHSH (make sure 4.2.1 SHSH were saved, if not don't bother continuing)
Start TSS Server In tinyUmbrella (If you get errors read the help section or FAQs or comment below typically it fails because you have some kind of web sharing enabled... use google to fix it - your issue really not much I can do...)
Open iTunes
Select your iPad in iTunes
Shift(windows) / Option(Mac) restore -> Select the 4.2.1 ipsw
Restore SHOULD error with a 1013 error. Click the "ok's" and close iTunes.
With your device sitting with the usb pointing to iTunes (recovery mode), enter DFU by pressing and holding HOME + POWER until your screen goes black... then after a count of 2 (1.5-2 seconds) let go of POWER but keep holding HOME. If all goes well you will see nothing but a black screen. If your timing is even a LITTLE bit off, you will see the apple logo and your iPad will boot back into recovery mode... Go back to step 10 and try again.
You're now in DFU mode. click 'Fix Recovery' on tinyumbrella
Click 'Yes'
Your device's screen will turn white and you'll see debug boot information.
Eventually your device will boot into the activation screen.
Congrats. You're on 4.2.1 (again)?
Hope that helps...