I think this was for using newer PS2 keyboards with even older PCs. I believe you need a specific Amiga adapter that will remap PS2 keys to Amiga keyboard standards.
Doesn't the A4000 use a mini DIN connector for the keyboard? This might allow an A4000 keyboard to be connected to an A2000.
EDIT: Without verifying the power and gnd pinouts of the PS2 standard versus the Amiga, I wouldn't chance this.
