What do you have inside the A4000?
My advice: strip the machine to the bare motherboard, test the PSU. Add the CPU board, test again. Add the Floppy drive, etc.
A bad cap in the motherboard can make a short-circuit who simply prevents the PSU to start (they have a "crowbar sensor" to avoid destroying everything when a short is detected).
Also a bad floppy or bad keyboard can make funny things to circuit.
Simply test one thing after another. Boring, yes, but the only way to discover where the problem lies.
