The piece that I've been most excited about in Flex 4 is the faster Flex compiler. So I was interested to see Enrique's post on some of the speedups. To quote, "The performance test suite went from full compiling in four minutes with Flex 3 to two minutes with Flex 4." I expect it to be even faster when the final version is done, and this will be immensely helpful to large projects, like the ones I work on at Brightcove.
Another article on the subject in the Notes on Compiler Improvements from the Gumbo specs. It's a low-level look from a compiler guru at Adobe who's looking into how to speed up the ActionScript compiler. Most of it won't be of interest if you haven't looked into the compiler source code, but the graphs at the end provide some insight into the progress.
For some Flex 3 compiler information on this blog, you can check out A Faster Flex 3 SDK and Flex Compiler Resources.