It has been quite some time since I did an ATX power supply hack, but if I remember rightly, Pin 8 (grey) is the equivalent of the A4000 "Power good" signal. Just connect it straight up.
You will also need to pull pin 14 (green) to ground with a switch. The idea is that the ATX power supply is actually powered up at all times, and this switch actually beomes the power switch that turns on the Amiga.
Just be careful to bear this in mind when working on your A4000 - I fried a power supply in a PC once when I disconnected a hard drive power connector, with the computer off, but the power was still connected to the mains.
David.
