The Flex 4 SDK provides a number of amazing features that provide even more flexibility and control for developers. If you are currently using Flex 3 components it is still a good idea to download the Flex 4 SDK and begin compiling your Flex 3 applications using the Flex 4 mxmlc compiler, which has also been improved. Using the compatibility-version feature of the Flex 4 compiler, your Flex 3 applications will not change in appearance.
Flex 3 and Flex 4 components can be used together, and many Flex 4 components provide a greatly improved experience for developers. Implementing custom skins and using states has never been easier, and a large number of other new features will really enhance your applications.
I’d encourage everyone to download FlashBuilder 4 and start using the Flex 4 SDK and feel free to post questions or comments here and I will answer them as quickly as possible.