@olegil

A4000T is an AT form factor and can use the pins on the current ATX power supplies w/o the need of rewiring.

A500 use a standard case? Yes, the standard Commodore A-500 case fits nicely around it.

@olegil

AFAIK the A500 doesn't even come clost to any standards. The A4000T on the other hand uses an AT form factor motherboard and an AT power suply. I'm not sure how incomaptible this is with modern ATX systems, enough to cause a major headach I should think.
If you are thinking of puting an ATX mobo+PSU in an AT case first you have to see if the power supply will fit, I suspect not without a couple of modifications to the monting brackets, next you need to check that the motherboard would fit and that the mounting holes line up or you will need to drill new mounting holes and possible (re)move ny obstructions. NExt up is the backplate, I bet Commadore/Amiga Technologies used a non-standard backplate, I don't know if it's removable or even the standrard size/shape or if ATX backplates fit AT cases. You may have to chopup an exisiting one or even craft your own. Lastly check that those slots on the back line up and that the screwhole to attach the cards to the case fit and voila one ATX case in an AT amiga :D