In short What are the things that make QT4 BETTER than QT3?

[Brandybuck]

Thereare a lot of changes between Qt3 and Qt4. The main change, however, is a complete refactoring of the libraries. Painting is more efficient, collections are more efficient, views are more efficient, etc. Even the API is easier to use and understand.

But you can't sell the customer on refactoring alone. You need new bells and whistles to entice the purchasing managers. So...

• Model/View classes. Handle large amounts of data, multiple views on a model, delegates, etc.
  
• Arthur painting engine. Transparency, double buffering, anti-aliasing, etc.
  
• Revamped rich text engine. Cursor based editing, structured documents, etc.
  
• QGraphicsView. This is QCanvas++
  
• Java style iterators, plus full STL compatibility.
  
• Signal/slots across threads, and other improvements to multithreading.
  
• The library is split into multiple modules (core, gui, network, xml, etc).
  
• Easier drag and drop, improved UI use, QDoubleSlider, etc., etc.