Microsoft blames common characteristics in Silverlight 4 and it's predecessor for the documentation overlaps. This begs the obvious question, why release a new version if it's som similar to the old one.
"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," revealed a member of the Silverlight team.
Well don't expect this to be resolved anytime soon. Perhaps next time the Softies will get the documentation squared away first.