The MSDN Online documentation for Silverlight 4 is now live: http://msdn.microsoft.com/en-us/library/cc838158(VS.95).aspx
If you have been looking at the documentation for the last 6 months or so, you may be used to seeing two entries in the MSDN Table of Contents at this spot: one for Silverlight 3, one for Silverlight 4 (Beta or RC). Now, there is just a single "Silverlight" node.
Generally the information in documentation is written as it applied to Silverlight 4. In cases where there is a difference in behavior if you are still targeting Silverlight 3, the difference is often called out in a separate section of the topic. Or in some cases there are entirely separate topics for a subject area as it applies to Silverlight 3.
In the Table of Contents and other navigation features such as members tables or namespace lists, the API is listed as it exists in Silverlight 4. If you want to know whether an API is new to Silverlight 4 (and therefore is not available if you are targeting Silverlight 3), check the Version Information section that appears towards the bottom of each managed reference API topic. If you don't see "3" listed somewhere after "Supported in:", then the API is new to version 4.
For anyone that is upgrading a project from version 3 to version 4, a very important resource is this topic: http://msdn.microsoft.com/en-us/library/cc645049(VS.95).aspx
If you have bookmarks or are following links to content that used to be part of what we published for Silverlight 4 Beta or RC, that content is still there, and is mostly current for Silverlight 4 RTM. However, the TOC won't synch up, which is sort of a giveaway that you are in this situation. Another way you tell if you have such a link is if you see (vs.96) as part of the URL you land on.
In these cases, you can make sure you are looking at the most current Silverlight 4 RTM topic by clicking the link that says "Microsoft Silverlight" in the little box on the right that says: "Other versions are also available for the following:". Note that these instructions are for the "Classic" view on MSDN; there are ways to jump versions on Lightweight view too but it is MUCH easier to do this when the view is Classic.
The material where (vs.96) is in the URL is also subject to being overwritten for whenever it is that we publish material for the NEXT beta of Silverlight. When that happens, it is likely we will put a branch into the Table of Contents again, so that an anyone trying to develop to Silverlight 4 RTM doesn't get caught up in beta-specific information that won't exist on their version.
PS: if you really need an isolated docset that is written just as it was for the Silverlight 3 era, you can use the offline CHM: http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=0a9773a7-c854-41fa-b73d-535abfb73baf