If you need to downgrade back to 3.1 here is what I did.
is some instructions for Windows users. I have not tested this as I did the revert on a Mac.
On a Mac you'll want to do a search for ".IPA" and should find the 3.1 version somewhere. You can also try downloading it from here
if a search doesn't return anything.
After you have the file open iTunes and go to File > Add To Library . Navigate to the .IPA and hit ok. Then you'll have to sync the app to your device and you should be good to go.
Unfortunately I lost all my books/settings when downgrading but at least Stanza is working again ....