Flash Builder 4 will be a paid for product (just like Flex Builder 3). We do offer free educational licenses though. The Flex 4 SDK will remain free and open source.
Where can I get the free student license for Flash Builder 4? I'm very interested in using Flex for some of my HCI classes. Thanks!
I believe the EDU serial you get from the site for FB3 can be used to
get the trial extension for FB4 beta. Once we release FB4 we'll
continue to have the free EDU system.