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