Microsoft made available both Silverlight 4 Tools for Visual Studio 2010 and Silverlight 4 SDK for developers relying on Silverlight 4 for their projects. The tools bring to table add-on and pre-requisite files tailored to VS2010, streamlining creation of Silverlight 4 and RIA Services apps.
Fact is that those integrating Silverlight 4 Tools and Visual Studio 2010, no longer need standalone SDK. This because S4T for VS2010 contain S4 SDK along with S4 developer runtime; an update for VS2010 and Visual Web Developer Express 2010 (KB982218); Silverlight 4 Tools for VS2010; WCF RIA Services V1.0 for Silverlight 4 and F# Runtime for Silverlight 4.
Download: Silverlight SDK 4.0.50826.0: EXE or Download Center
Silverlight Tools for Visual Studio 10.0.30319.352 / 30730.126 / 21124.00