4 pin can provide a pwm signal to control pwm fans. Some motherboards also can use voltage to control non pwm fans. You can usually plug a 3 pin fan into a 4 pin socket and the otherway around you just loose the pwm functionality
Thanks Bud this is what I thought. Cheers --- I am here: http://maps.google.com/maps?ll=50.678967,-3.491281
PWM = digital fan speed control at constant voltage using pulses of varying width. Much more efficient than changing the voltage level to control fan speed. Although 4 pin CPU fan headers can accomodate 3 pin fans, quite often they don't regulate the speed unless you use 4 pin fan. This is the reason I ditched all of my 3 pin fans last week and purchased 4 pin PWM fans that can plug into each other serially for up to 5 fans. Thus you can drive 5 fans on a single PWM fan header. Fitting the new PWM fans has dropped my liquid cooled GPU temps by 10c to = 31c full load in Battlefield 3. Bottom line? Ditch any 3 pin fans and go for Arctic F12 PWM... http://www.kikatek.com/product_info.php?products_id=167301&source=froogle You can chain up to 5 of these togther without having to buy a fan cable splitter. 74 CFM per fan with very low noise.
