- Big jump forward in the goal towards Designer to Developer round tripping. A designer can use Adobe Catalyst to design the app AND it's interactions, while the developer plugs in the logic and they can work and in hand... whereas normally - if a designer makes a big change, it's quite the ordeal for the developer to go back to incorporate the new design.
- View states were completely redone to actually be easy to do.
- Flex 3 Halo components are super heavy weight (a ton of properties to control the look). Flex 4 introduces Spark components which are much more lightweight and simple and meant to provide basic building blocks.
- Along those lines, Spark components are highly skinnable. In Flex 3, you could do some CSS, but if you really wanted to go to town you had to do programmatic skins which were totally not worth it.
There's some other stuff, but those are some of the big highlight changes.