The position of the PSU in the A4000T is not really clever at all. It is mounted so that you lose a good amount of space above it. That and the difficulty accessing the innards makes the A4000T case not such a great design functionally. I don't think that installing a smaller PSU will help.
AT PSU: I got mine from a local computer fair a while back, for £6. That vendor isn't there anymore, so online will have to be the next step. But....I would go the ATX route instead. Get rid of the AT, and then open up your options considerably.
What would be the advantage of using an ATX PSU over an AT one in the 4000T?
