To answer your question, no.
For my person experience, I did try to downgrade my iPod touch 4G (it came preinstalled with iOS 4.1 but i made a mistake to upgrading it to 4.2.1) to 4.1 and it didn't work. I did manage to get a tethered downgrade, but that was buggy and so i restored to iOS 4.2.1.
So, unless someone learns how to generate random SHSH files for a specific device and trick iTunes into thinking it's signing 4.1 when it's not, your out of luck :'(