How to downgrade from iOS 4.3.5 to iOS 4.2.1?
Firstly, search Google for "download ios 4.2.1" and download the correct version for your device. If it's in a zip, unzip it, but you should end up with an .ipsw file.
Open iTunes and select your device on the left. Shift-Click (or Option-Click on Mac) on 'Restore' and select the file you downloaded.
This should allow you to install whichever version of ios you have the .ipsw for
If you have not got your SHSH for iOS 4.2.1 (it could be saved only if you have ever jailbroken your device), then Apple will reject downgrade the version of iOS.