http://doc.trolltech.com/qq/qq09-qt4.html
We are putting an enormous amount of effort into Qt 4, and we are very excited about the new capabilities and technologies that we are building into it. Qt 4 represents a significant leap forward, and we want to be sure that you are happy with the work we are doing. For this reason, we plan to release a Qt 4 Technology Preview by the summer, with as much of the functionality we've described here in place as possible. We hope that you will download and try out the Technology Preview and give us your feedback so that we can incorporate your input into the Qt 4 beta that is planned for the second half of 2004.
Seamless tools integration
Modern consumer-style user interfaces
A new paint engine
A new text-rendering engine
A model/view framework
A new set of containers
Extensive support for multithreading
Numerous API improvements
A new docking architecture
Multiplatform accessibility
Better support for non-GUI applications
Significant performance and size improvements