I haven't done much with computers for the last year, so maybe it is new, but I have never seen a 4 pin fan connector that had speed control on the 4th pin. the only 4 pin fan connector I have ever seen is the kind that plugs into a regular 4 pin power supply connector. these have 2 grounds a 12v and a 5v pin.Only the 12v pin and 1 ground are actually use.
Are you sure you are not looking at a Aux 12v power connector for the motherboard?
Speed control dosen't require an extra pin, because it is done by reducing the input voltage before it gets to the fan. The 3rd wire on a 3 wire connector, is for the tachometer.