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 SourcesThis topic describes how to create parsers for container formats and describes delivery mechanisms not natively supported by Silverlight.
Delivering and Accessing Media ContentThis topic describes steps that you need to perform to deliver and access media content in Silverlight.
Troubleshooting Media IssuesThis topic describes how to diagnose and solve media issues in Silverlight.
Updated: Digital Rights Management (DRM)New Output Protection section.
Updated: Supported Media Formats, Protocols, and Log Fields
Updated: MediaElement States