Marcop300,
I believe you can downgrade the firmware. Just follow the "Upgrading the Firmware" instructions in the user guide without any hard disks installed with the 1.3 version you want.
The system operating system, "the firmware", is actually run from the hard disks after the system is initialized. The firmware is uploaded to the hard disks at initialization.
In your case with 1.3 on the SS4000 after the "downgrade" and 1.3 on the hard disks, it'll just boot normally.
John