how do i undo "forget this device" for a bluetooth connection from my iPhone 4 to a Panasonic sound system?

There's no way to "undo" this without restoring to a backup made while this pairing was still on your phone. (Paired bluetooth devices are included in the iPhone backup.) Restore to a backup will revert all data and settings to those contained in the backup, not just the pairing settings. If you wish to do this, save any data since your last sync/backup, then do the following:

1. Without connecting your phone to your computer, open iTunes on your computer and go to Preferences, on the Devices tab check "Prevent...from syncing automatically".
2. Connect your phone to your computer and right-click on its name when it appears in iTunes on the left sidebar, select Restore from Backup, choose your most recent backup to restore from.
3. When done, go back to iTunes>Preferences>Devices and re-enable automatic syncing.

Edit: this won't work if you have already synced again since this happened as the sync process will have already overwritten your earlier backup.

Ok. Just a thought, but does your Panasonic sound system also have an unpairing setting? If so, you may need to unpair the iPhone on your sound system too (i.e., forget the iphone) before it will allow it to re-pair. Also, not sure if this applys to bluetooth or not but you might try Settings>General>Reset>Reset Network Settings on your phone. (Note: this will remove passwords for previously joined wi-fi networks so you will need to re-enter these again if you choose to try this.)