Silverlight 3 projects are supported with the Visual Studio 2010 Release Candidate build – however Silverlight 4 projects are not yet supported.
We will be adding Visual Studio 2010 Release Candidate support for Silverlight 4 with the next public Silverlight 4 drop.
If you are doing active Silverlight 4 development today we recommend staying with the Visual Studio 2010 Beta 2 build for now.
Tim Heuer has also blogged about this limitation here.
Have a great day,
Visual Studio Cider Team