• new set of skinnable Spark components and skinning architecture which leads to better separation between data and view
• introducing FXG declarative syntax for defining graphics
• two way data binding

But if you want to explore full list of changes and improvements to framework, there is a “Flex 4 features and migration guide”, which could give you more information about it.

New set of Spark component architecture defines entirely new way of building application view. Now view and data are separated, and each of functional component can be skinned to look completly different than “default” one. For example you can skin radiobutton to look like button, but still have radiobutton functionality – single select or grouping. This feature is extremely useful, and makes life of developer much easier.

Spark components also adds some new features e.g better handling of text by RichText and RichEditableText controls which enables developer to handle text more efficiently and easily. My personal foe – Repeater could be replaced by Spark DataGroup component which behaves far more intuitive  and of course is suited to new skining architecture

Flex 4 introduces also FXG declarative syntax, it is similar to Degrafa, and in simple words enable programmer to create vector graphics by writing piece of MXML code. This alone is not a big deal, but please remember that there is a data binding mechanism – and having those two together implementing a custom bar progress indicator or some fancy buttons is much, much easier.

Finally we have databinding that works both ways, so when connected to e.g. RichEditableText you will got property synchronized with it’s text. The only thing to remember is to have property that need to be bound declared as public.

Flex 4 is a big step ahead, it resolves a lot of problems that was a pain in Flex 3. I will encourage everyone to use it since it is far better than Flex 3, in my opinion, and could save some development  time especially in skinning of application. Together with Catalyst and Flash Builder it will create a powerful platform for building RIA applications.