So in case you haven't heard, Adobe has released the beta version of the next version of Flex Builder. The new IDE is going to be called "Flash Builder 4" and is still built on Eclipse. Along with the rest of the world, I began playing around getting used to the new interface. Already, I've noticed some MAJOR improvements not only in the full CSS support of Flex 4 from within the IDE, but a few minutes ago, I was BLOWN away at some other cool new features.
The first thing I noticed was the improved intellisense! Now, when you ctrl+space you not only get a list of properties/events/effects, but you actually get them in groups. Initially pressing ctrl+space will give you a list of component properties, pressing ctrl+space again will bring up events, again for effects, again for styles. How amazingly cool and SUCH a time saver. Having come along all the way from Flex 2, I can not tell you how many times I've had to trudge through the intellisense menus looking for whatever I needed at the time. Now unlike some, I don't keep every property, event, etc in my memory and being able to find my item quickly is worth its weight in gold. Very nice!!
The other REALLY cool thing I noticed is that Flash Builder 4 will now stub out your events for you. WONDERFUL!!! I came from Visual Studio and losing this type of functionality when moving to FB2 and FB3 was disheartening. Again, I don't memorize every single method signature, and have had to spend countless hours going back to the livedocs having to look them up... especially when they are not used very often. Now, in the property manager, you can double-click an event and Flash Builder 4 will automatically stub it out for you. Gold Star to Adobe! 
You can now also click on a component class name like "Label" and the livedocs will open within Eclipse to allow rapid reference!!
I'm sure there are developers out there who won't find these features very valuable, but I for one have already become VERY excited at the prospect of not having to constantly keep a browser window open to livedocs. The next wave of Flex development is already VASTLY improved over Flex Builder 3 with the modifications to Flash Builder 4.