When I go to download the Adobe Add-Ons for the release version of Flex 4, I get sent to http://fpdownload.adobe.com/pub/flex/sdk/builds/flex4/flex_sdk_18.104.22.168595_add-on.zip, which gives me a 404 error. Earlier versions seem to work fine.
I can see that too on http://opensource.adobe.com/wiki/display/flexsdk/Download+Flex+4 and I've passed it along to our builds/web people who will take a look.
Kudos to the builds/web people, it's working now. Thanks.