well the molex is already giving 5 and 12v so all you need is a regulator. radio shack sells dials that regulate voltage by 1.2-2.5W, those should be enough to controll a bit of speed and there cheap. just place 4 of those in a PCB, solder a molex on the back and 4 leads coming out from each controll knob. the knob's have 3 pins, hot, ground, and lead out that recieves the modified voltage. from there the 4 plugs will hold a common ground back to the molex.