License
Qt is now released under the Lesser General Public License meaning it can be freely linked into commercial applications.
New
Read What’s New or a more detailed description to see what’s changed since Qt 4.4 (released in May, 2008).
If you haven’t upgraded to Qt4, here’s what changed since Qt 3.3.4 in June, 2005.
Latest WebKit (aka the html display used by Apple’s Safari browser) including support for Netscape plug in so, for example, Flash can be embedded. Includes HTML 5 and CSS animation.
XSLT support (XML translation)
QtCreator is a Visual Studio like editor that works cross platform and incorporates debuging support and Qt Designer (a layout editor).
Mac OS X Cocoa support (latest Mac graphics library). Enables 64-bit support.
Performance
Read either Improved Performance or a white paper with more details and it requires registration so they can email it to you.
FileDialog is significantly faster (60x in one case)
QGraphicsView optimized
Clipping sped up.
Added SquirrelFish javascript engine which is much, much faster.
Added QtBenchLib to make application performance testing easier.
Noteworthy
A few other noteworthy changes:
Networking classes have better proxy support and detection of system settings
Writing OpenDocument formatted text
Stylesheets now work on Mac OS X
Better translucent windows