Basically, all you need to do is to move the drive select lines up by one in the CIA that handles the floppy drives.
CIA - MOBO
==========
DS0 - DS1
DS1 - DS2
DS2 - DS3
Unfortunately you can only connect three drives to the machine in total then.
Also, it's not quite that straightforward as the CIA is soldered onto the mobo. You'll need to desolder and lift up the legs and use wires to reconnect them.
