Adobe released Beta 2 of Flash Builder 4!
You can download it here.
The beta 2 release of Flash Builder 4 will run for 60 days from installation before timing out. You may continue your use of beta 2 after the first 60 days by entering a valid Flash Builder 4 extension serial number. If you own a copy of Flex Builder 3 and did not receive an email with an extension serial number, you may request one online by entering your valid Flex Builder 3 serial number.