First of all you need to have latest version of itunes.
Well don’t panic since this is a really simple thing to do, first, You download an image of iOS 4.2.1 from web and you can force the iphone to use the download as a recovery in itunes. you want to restore into. so get the IPSW (iPhone Software) and then move on to the next step.
Now that we have the IPSW ready simple plug your iPhone to your computer, and when it shows up on iTunes Hold down the Shift key and press Restore. You will be asked for the location of your desired IPSW file, simply select the one you just downloaded and click Open. Your iPhone should now be restored without problems to that version. This is also how you could downgrade your iPhone OS if you accidentally updated it.
I will try find the site for you but last time I looked there were loads of them.
you can use tiny umbrella to get a .shsh and then let itunes updates to 4.2 then downgrade it to 4.1.
Following are very useful links to solve the problem.
Link to Windows Version of redsn0w: