John Papa explains what’s new in Silverlight 4 in 12 minutes up on Channel 9.
The video can be found here: http://channel9.msdn.com/learn/courses/Silverlight4/Overview/WhatsNew/
A word-doc format whitepaper going into details of these new features can be found here: http://go.microsoft.com/fwlink/?LinkId=177698
Sascha Corti aka. TechPreacher 