The new Flash Builder 4 (formerly Flex Builder) is out in its Beta. It has loads of new features. Some of them are -
Developer Productivity – Event Handler generation, Getter/setter generation code indentation, file templates, etc..
Debugging – Contitional Breakpoint, Run to line, etc..
Working with data in a brand new way (Data Centric Development) – Connecting to HTTPService, WEBService, PHP, ColdFusion, BlazeDS, LCDS has been made so easy.
Flex Unit Support.
and many more….

Useful links -
Download – http://labs.adobe.com/technologies/flashbuilder4/
What’s new – http://www.adobe.com/devnet/flex/articles/flex4builder_whatsnew.html
List of Articles – http://sujitreddyg.wordpress.com/flash-builder-4/
Don’t forget to log the issues/enhancement you encounter in the bug-base here – http://bugs.adobe.com/flex

Go ahead, download it and give us feedback to make it better.