When I had a real 4000D PSU, the fan was sucking air from the inside of the chasis and forcing it out the vent in the back. I had two problems: that the 4000D fan is throttled by a resistor, and the cards (X-Surf and P-IV) were generating too much heat and not enough airflow.
Resolving the first did not fix the second. I bypassed the resistor to increase fan speed (replacing the fan helps, too, as those things get ratlly and noisy in old age, as many of us do,) but the case remained hot to the touch. My next magic trick was to put a vented spacer in the back, which reduced the heat in the case dramatically.
The spacer was actually the mounting bracket for one of those slot fans which tend to seize up after a few days of use. I do not recommend using a slot fan as it will fight against the exhaust fan in the PSU. I tried one at first and it helped with the card area, but the hard drive side of the case stayed really hot. Designs I have seen of the 4000 show the airflow is supposed to come in across the cards, flow around the front, across the CPU card, then out the PSU.
In the end modifying the PSU did not matter much as I replaced it with a min-ATX PSU, which has worked wonderfully (I believe I have a picture in my gallery.)
For what it is worth, the original PSU fan modification was not unbearably loud -- it was not silent, but it was not obnoxious. The mini-ATX is effectively silent.
