Via the Official Adobe Flex Blog.

Flex 4 SDK and Flash Builder 4 final releases are here...
By Andrew Shorten on March 21, 2010 11:15 PM | No Comments

On behalf of everyone on the Flex and Flash Builder product teams, it is with great pride that we can announce that the final versions of Flex 4 SDK and Flash Builder 4 are available for download today!

We've been working hard on these releases to make the Flash Platform the best RIA development platform ever and one that you can confidently bet on when you are asked to create your next generation applications.

Get Adobe Flex 4 Now