Sorry, due to an old forum bug missed your reply yesterday, you can re-enable disk3, but since the array was mounted without the old disk parity won't be 100% in sync, so there could be issues replacing the failing disk, though there's not much to lose by trying:
-Tools -> New Config -> Retain current configuration: All -> Apply
-Check all assignments are correct and assign any missing disk(s) if needed, including the old disk3
-IMPORTANT - Check both "parity is already valid" and "maintenance mode" and start the array (note that the GUI will still show that data on parity disk(s) will be overwritten, this is normal as it doesn't account for the checkbox, but it won't be as long as it's checked)
-Stop array
Array is now as it was before, except like mentioned parity won't be 100% valid, but with xfs this is usually recoverable, you might need to do a filesystem check once you replace disk4, especially because the fs on the actual disk is already showing issues, most likely because of the bad sectors.