The documentation on Adobe Flash Builder Extensibility API Reference has been updated for Flash Builder 4 and is now available. The documentation also contains links to videos and examples that show how to extend Flash Builder.
Use Flash Builder Extensibility to customize Flash Builder according to your development needs. You can modify the behavior of Flash Builder and specify how custom controls appear in Flash Builder Components view and Properties view.
Using the Design View Extensions for Flex 4, you can now feature custom components with the Flash Builder native components. This allows you to enable features, such as drag and drop, for custom components.
Code Model and Design Model have been updated for Flash Builder 4. Design View Extensions Kit for Flex 4 is now public. This kit contains example projects that show how to incorporate custom components into Flash Builder.
There is also an Early Access version of DCD Extensibility. DCD Extensibility allows you to update Flash Builder data services tools to access additional services.