Many (perhaps most) A4000s were shipped with improperly-wired floppy cables. These cables had wires 3-5 twisted, instead of wires 4-6. These cables will work fine for a single drive, but will not properly connect a second drive. To use two drives that are both jumpered as DS0, the floppy cable should have both wires 4-6 and 10-12 twisted. A cable with only wires 4-6 twisted will require the drives to be jumpered as DS0 and DS1.
An improperly-wired or failing floppy cable can cause a high-density drive to only work with double-density disks.
Pin-outs for the internal floppy connector are shown in Internals/Connector
Pin-Outs/Internal Floppy Connector Pin-Outs.