This will install the developer runtime of Silverlight 4, the Visual Studio project support and the Silverlight 4 SDK. If you are developing Silverlight 4 applications, this will be the minimum you want to install.
If you installed the tools above, you will get the developer runtime and there is no need to install again. These downloads are being made available for test machines for the Windows and Mac platforms for your applications.
The toolkit provides support for Visual Studio 2010 and Silverlight 4 development. The Toolkit provides numerous controls for your application and source code is also included for these controls using an Open Source license.
Microsoft WCF RIA Services simplifies the traditional n-tier application pattern by bringing together the ASP.NET and Silverlight platforms. The RIA Services provides a pattern to write application logic that runs on the mid-tier and controls access to data for queries, changes and custom operations. It also provides end-to-end support for common tasks such as data validation, authentication and roles by integrating with Silverlight components on the client and ASP.NET on the mid-tier.