Silverlight 4.0 and Visual Studio 2010 Released Candidate

Only Silverlight 3 projects are supported with the Visual Studio 2010 Release Candidate build – however Silverlight 4 projects are not yet supported.
If you are doing Silverlight 4.0 development, it is recommended to stay in Visual Studio 2010 Beta 2 build for now.

For more info you can check the links below:
Quick FAQ on Visual Studio 2010 RC …
VS2010 RC and SL4