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
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Read the complete post at http://blogs.msdn.com/b/silverlight_sdk/archive/2010/08/24/silverlight-4-and-silverlight-3-msdn-documentation-are-combined.aspx