The newest version of TinyUmbrella added side-benefit of allowing you to update to 4.1 without the fear of baseband update. Of course, in order to downgrade back to 4.0.x you need to have your SHSH saved on Cydia or locally.
You need to have your iOS 4.0.1 SHSH saved on Cydia or locally.
This will only work while Apple is signing 4.0.2 SHSH requests.
You can downgrade to iOS 4.0.1 anytime, only if you satisfy first point mentioned above.
Step One : Download latest version of Firmware Umbrella (Download Here)
Step Two : Close iTunes, Skype, Messengers or any other background application.
Step Three : Launch Tiny Umbrella and connect your iPhone 4
Step Five w launch iTunes and restore to iOS 4.1 (download here) Note : do a *Restore* not *Update*
Step Six : Cross your fingers and wait for iTunes to end up with error 1004. This is perfectly normal and has to do with the baseband not being updated.
Step Seven : Simply ‘Kick Device Out of Recovery’