There is quite a bit of overlap between developing for Silverlight 3 and Silverlight 4 and so in order to not duplicate content in different locations on the web, both versions are covered in the same set of MSDN documentation. Below is a list of tips for finding version-specific information:

• For a general list of changes between Silverlight 3 and Silverlight 4, see Ensuring That Your Silverlight Applications Work with Silverlight 4.
• You can see what versions of Silverlight an API is supported in by scrolling down to the Version Information section of the API document.
• For API supported in both versions of Silverlight, version differences (if any), are called out in version information notes in the remarks section of the API document.
• Version notes are also available in overviews, tutorials, etc. In a few cases, there are separate versions of the same document (e.g. Digital Rights Management (DRM), versus Digital Rights Management (Silverlight 3)). This is done when version behavior for a given scenario or feature differs too much for version notes to cover in a single document.