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#21
The 4th wire is for PWM control of the fan, but only if it is connected to a 4 pin fan header. It won't affect anything if it isn't connected, and yes that 4 pin connector will go on a 3 pin header. In fact the AC Freezer comes with the same 4 pin connector and I used it for a couple of years on a 939 board that only had a 3 pin header. The only thing that happens when it isn't connected is that the fan will run at full speed, without the option of having it controlled by the bios.