Then I think your best bet it to use old disk2 and force Unraid to rebuild disk4, parity still won't be 100% in sync, so some fs corruption is possible, you can do that by:
-Tools -> New Config -> Retain current configuration: All -> Apply
-Check all assignments and assign any missing disk(s) if needed, including old disk2 and new disk4, replacement disk4 should be same size or larger than the old one
-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
-Unassign disk4
-Start array (in normal mode now), ideally the emulated disk4 will now mount and contents look correct, if it doesn't you should run a filesystem check on the emulated disk
-If the emulated disk mounts and contents look correct stop the array
-Re-assign the disk4 and start array to begin.
For now keep old disk4 intact in case it's needed.