If you know from what version you upgraded the ZD then you can directly downgrade and ZD will automatically restore the backup from last upgrade.
Below are the steps.
1- Make sure you know from what version you upgraded to 10.4 (keep in mind that downgrade without factory reset only works when you downgrade to very last version running on ZD). 2- Now go to support site and download the firmware file of same version(one which was running before the upgrade to 10.4).
3- Now to go Administration >> Backup/restore >> create a new backup on 10.4 version, just in case needed. 4- Now go to Administration >> Upgrade >> choose the downloaded file for downgrade >> complete the downgrade. 5- If correct firmware version file was uploaded then system will automatically restore the last backup file. If not, then system will factory reset and you have to reconfigure it from scratch.