Final answer:
The fourth pin on a 4-pin KK Molex fan connector is the PWM pin, used to control the fan's speed.
Step-by-step explanation:
The fourth pin on a 4-pin KK Molex fan connector is the PWM (Pulse Width Modulation) pin. It is responsible for controlling the speed of the fan by varying the voltage supplied to it. By adjusting the duty cycle of the PWM signal, the fan's speed can be increased or decreased.
The other three pins on the connector are the +12V (power supply), ground, and tachometer pins. The +12V pin provides power to the fan, the ground pin completes the electrical circuit, and the tachometer pin sends back a signal indicating the fan's rotation speed.