The Silverlight 4 documentation has been updated--both online MSDN docs and the offline CHM. The CHM is Silverlight only and is great for fast, offline access.
There are numerous tweaks and small changes to the docs, but here are a few of the new and updated topics:
Implementing MediaStream Sources
This topic describes how to create parsers for container formats and describes delivery mechanisms not natively supported by Silverlight.
Delivering and Accessing Media Content
This topic describes steps that you need to perform to deliver and access media content in Silverlight.
Troubleshooting Media Issues
This topic describes how to diagnose and solve media issues in Silverlight.
Updated: Digital Rights Management (DRM)
New Output Protection section.
Updated: MediaElement States