You won't be able to determine the RPMs of the fan through the 4-pin "Molex" adapter. If you look at your CPU fan, it has a special 3 pin plug. Only these kinds of fans can have their RPMs monitored.

As for adjusting the speed of the fan, !mitationRust got it right. A rheostat / fan controller can lower the voltage being applied to the fan and make it spin slower with the reduced power. I doubt the RPMs could be monitored through the Molex plug, but I could be wrong...

A controller such as this can allow you to adjust the fan speed...but it can't tell you the fan speed: http://www.newegg.com/Product/Produc...82E16813999504