Flex Builder was rebranded Flash Builder, and you can currently download a trial version of Flash Builder 4 at http://labs.adobe.com/technologies/flashbuilder4/. Unfortunately, the included Flex 4 SDK with the latest Flash Builder 4 beta is a bit older, so you may want to download a newer nightly Flex 4 build from http://opensource.adobe.com/wiki/display/flexsdk/Download+Flex+4. And you can find more information and rough milestone dates and specs from the Flex 4 (codename: Gumbo) from the Adobe Open Source site at http://opensource.adobe.com/wiki/display/flexsdk/Gumbo.
Personally I probably wouldnt try getting the Flex 4 SDK working in Flex Builder 3. I'd use the Flash Builder 4 beta instead. If you have a Flex Builder 3 serial number, there should be a way to extend the trial period of the Flash Builder 4 beta; https://freeriatools.adobe.com/flashbuilder4beta/.
There is also marginally more information at http://blog.flexexamples.com/flex-4-codename-gumbo/ (links to early Flex 4 documentation, public Flex bug base, etc).
Hope that helps,