As far as I can recall (will check when I get back home), the A4000 PSU is supposed to suck air in through the holes in the vicinity of the Zorro cards, then the air travels across the cards, along the raiser board, and the rounds it at the front, travelling across the drives and into the PSU, where the air comes out.
One solution to the cooling problem which I remember is to drill up more holes at the card cage.
