Here is an excerpt from the official blogpost, detailing where to go to learn more:
I like the Flex testdrive page that they put up as well.
As you can see, there are lots of great new features and enhancements in Flex 4 and Flash Builder 4 - too many, in fact to list here. We recommend that you read the "What's New in Flash Builder 4?", "What's New in Flex 4?", and "Introduction to Spark" articles, as well as check out the rest of the new content published on the Adobe Developer Connection site to learn more. Additionally, you can watch David Wadhwani, Vice President and General Manager of the Flash Platform Business Unit describe the new set of products being added to the Flex product family.
To commemorate the release, The Flex User Group, Singapore and Adobe Singapore have planned a launch event. Flash Platform Evangelist Harish Shivaramakrishnan will be flying in from Bangalore, India to introduce Flex 4 features, and Flash Catalyst. Hu Shunjie will also be discussing the new Spark component architecture. For detailed Schedule, head over to the official FUG blogpost here.
Attendees also stand to win a Flash Builder 4 Premium license from Adobe. Seats are limited, so go ahead, and register for your seat now.