4000 series parts have the advantage of lower power consumption, wider range of supply voltages (3 V to 15 V), and simpler circuit design due to the vastly increased fan out. However their slower speed (initially about 1 MHz operation, compared with bipolar TTL's 10 MHz) limits their applications to static or slow speed designs. 

New fabrication technology has largely overcome the speed problems, while retaining backward compatibility with most circuit designs.