Can I replace 3pin fan by 4pin Fan

Solution
A 4-pin will plug into a 3-pin header, and vice versa. The 4th pin is for a PWM signal which allows you to control the fan's speed. A 3 pin header cannot control the fan speed on a 4-pin PWN fan, and a 3-pin fan plugged into a 4-pin PWN header cannot have its speed controlled.
A 4-pin will plug into a 3-pin header, and vice versa. The 4th pin is for a PWM signal which allows you to control the fan's speed. A 3 pin header cannot control the fan speed on a 4-pin PWN fan, and a 3-pin fan plugged into a 4-pin PWN header cannot have its speed controlled.
 
Solution