Microsoft has released the SDK version 4.0.7. on January 15th .
The Microsoft Dynamics CRM 4.0 Software Development Kit (SDK) is for developers, system customizers, and report writers. It contains all new information about creating plug-ins, working with custom workflow activities, using the new Web services, using new data management features, and much more. Everything developed outside the SDK is NOT supported!
The SDK can be downloaded here.