That has happened!
Initial point of Silverlight 4 web portal can be found here.
Silverlight 4 Toolkit (April 2010) can be downloaded here. Silverlight Toolkit April 2010 changes can be viewed here.
Silverlight Toolkit April 2010 does not support Silverlight 3 and Visual Studio 2008. Use November 2009 release with Silverlight 3 and Visual Studio 2008.
Microsoft Expression Blend 4 RC has been released too. It can be found here.
Great article about Silverlight 4, installation and other related things is here