For the latest information on the next major release of Flex SDK and Flash Builder 4 (codenamed “Gumbo”), check out the Flex 4 page on the Adobe Open Source site: http://opensource.adobe.com/wiki/display/flexsdk/Gumbo. The Flex 4 page includes the latest milestones, as well as links to the various public specifications. Also, you can check out the Flex 4 page on the Adobe Labs site at http://labs.adobe.com/technologies/gumbo/.
To download nightly builds of the Flex 4 SDK, check out http://opensource.adobe.com/wiki/display/flexsdk/Download+Flex+4.
For the latest Flex 4 documentation, check out http://livedocs.adobe.com/flex/gumbo/langref/ and http://livedocs.adobe.com/flex/gumbo/html/ (as well as some Flex 4 beta chapters at the bottom of http://opensource.adobe.com/wiki/display/flexsdk/Gumbo).
If you think you’ve found a bug in the Flex SDK or in Flex Builder (any versions), you can file a bug in the public bug base at http://bugs.adobe.com/flex/.
For a list of Flex 4 specific examples on this blog, check out http://blog.flexexamples.com/tag/gumbo/. For an excellent walkthrough of migrating from Flex 3 to Flex 4, see Joan Lafferty’s excellent Adobe Flex DevNet article, “Differences between Flex 3 and Flex 4”.
Want a free version of Flex Builder 3? Check out https://freeriatools.adobe.com/ and see if you qualify! Then, head over to the Adobe Flash Builder 4 Beta Extension Request site to request a Flash Builder 4 beta serial number. (Flash Builder 4 beta extension serial numbers are available for all licensed Flex Builder 3 customers. This includes all commercial and education serial numbers.)