Hi,
If you are using player 10 ALCS swc and Whiteboard with Flex 4 and Flash Builder beta 2, it might throw an exception sometimes which has been fixed recently and will be updated in the next drop.
Player 9 ALCS swc should work fine. We haven't done testing extensively on Flex 4 for our framework, so if there are any issues that you come across, please report to us.
Thanks
Hironmay Basu