Unless you do not fear death and have electrical knowledge and experience in power supply conversions, I'd recommend buying a replacement from and having it installed by your local qualified Amiga dealer, to be on the safe side.
It would be a simple matter to open up your A4000 (power off!) and measuring the size of the power supply, then comparing it to a ATX power supply.
I can't tell you if it can fit, but since someone's apparently made adaptors, the chances are good that it will fit.
