This package is an add-on for Visual Studio 2010 Beta 2 to provide tooling for Microsoft Silverlight 4 and RIA Services. It can be installed on top of either Visual Studio 2010 Beta 2 or Visual Web Developer 2010 Express Beta 2. It extends existing Silveright 3 features and multitargeting capabilities in Visual Studio 2010 to also create applications for Silverlight 4 using C# or Visual Basic.
Silverlight 4 Tools for Visual Studio 2010 Beta 2 includes several features:
Support for targeting Silverlight 4 in the Silverlight designer and project system
RIA Services application templates and libraries
Support for Silverlight 4 elevated trust applications and other new Silverlight 4 features
This download will install all components necessary for Silverlight 4 and RIA Services development:

Silverlight 4 developer runtime Beta
Silverlight 4 SDK (software development kit) Beta
Update for Visual Studio 2010 Beta 2 and Visual Web Developer Express 2010 Beta 2 (KB976272)
Silverlight 4 Tools for Visual Studio 2010 Beta 2
WCF RIA Services for Silverlight 4 Preview