Pin layout is
1 = GND
2 = V+ Supply
3 = ?
4 = V+ Control
I breadboarded a simple circuit to allow me to control the fan speed. psu9.png
However, the big problem is the fan is NOISY! Really noisy.... like a hurricane.
I tried running the fan at 5 Volts Supply and that lessened the noise a bit, but still a bit noisy. The ideal voltage is about 3.3Volts for the fan to be quiet (and still be spinning), but at that speed it isn't moving much air.
So... I'll have to ditch the MacPro PSU fan and buy an after-market quieter fan.
One thing though, these MacPro fans are tough. I fed it 20Volts Supply and you think it's going to fly. Really LOUD... across the room LOUD. (If you ever had a serious KP on an Apple G5, and the fan revs up really loud, you know what kind of LOUD I'm talking about.)