For me Flash Builder is a lot slower than Flex 3. I´m developing the same AS3 project I used Flex 3 for. FB4 gets stuck once in a while. Most annoying thing is the autofill that 50% of time requires 2 times of "ctrl-space" to happen.
The same happens when I write a method like "private function onStuffHappened(event:Event):void", then push enter. Sometimes the enter is ignored. So annoying! I´m working on Compaq 6710b laptop, should be good enough for this stuff. With same computer it worked like a charm.
If this is the responsiveness of final version I´ll really have to re-evaluate FDT, though I´ve liked Flex3
FB4 is in a beta state, so let's wait until it'll it will be released or the first rc is available.