So what's new in DivX 4.0? Well, it's completely re-written to improve performance and visual quality. The code base is different from all earlier versions, including the open-source OpenDivX codec. Some new features:

Improved decoder performance
Higher encoding performance, almost 4 times faster than previous version
Multipass encoding and 2 different variable bit rate modes
Improved visual quality
Highly scalable codec from 16x16 pixels to 1920x1280 pixels (i.e. HDTV). All even integer dimensions are acceptable.
Support for popular encoding/editing applications
Plays all DivX(TM) content