This is a simple guide to Firmware downgrading with iTunes 8 and 9 without loosing Music from the iPod (this is NO warranty, backup your data if it's valuable to you!!!)
First you need the correct firmware file. You need to put the firmware file in a folder, then open iTunes, connect your iPod and go to the status page. Shift-Click on the "Check now" or "Update" button near the "Restore" button. Now select your firmware and wait until it gets downgraded.
Later I'll post a link to the actual .ipsw file for the 4g nano, don't have it at the moment.