OK, there seems to be much confusing info given here to the simple question.
The 3rd wire for fan is the Tach/speed feedback. Please refer to the Intel ATX standards ....
You can solder a wire onto the PCB to have this report the RPM to your computer or fan controller. However, it's been too long to remember which of the two is the tach. Search the web then ...
The 4th wire is PWM control. The Laing DDC pump series has a proprietary PWM (Apple, Dell, HP ... BIOS controlled PWM), thus it is useless to wire that solder point. A fan controller will do just fine with the two wire, but provides nothing other than trying to listen for the pump hum or gurggling. The tach wire to the fan controller is ideal to have.
Hope this helps,
BMaverick, meeting our DDC pump info needs