The GeForce4 4600 cannot be overclocked very well. She has a very fast RAM (2.8 if I'm not wrong) but for some reason you cannot get the same fps increase you can get overclocking the 4400.
Same thing happens for the GeForce3 Ti500 and Ti200.
Just FYI, the 4400 can be overclocked from 550 to 650MHz and more .... (the 4600 is clocked to 650).