Microsoft Silverlight 4 released about a month ago. Microsoft has now released their Silverlight 4 tools for Microsoft Visual Studio 2010 within the Microsoft download center Silverlight 4 Tools page.
This product is an add-on to Visual Studio 2010 that provides support and tools for Silverlight 4 development. You can use it either the Visual Web Developer 2010 Express edition or with the fuller versions of Visual Studio 2010. You'll be able to start using the Silverlight designer to target version 4, you'll get access to RIA Services application templates and libraries, and you'll be able to build elevated trust and out-of-browser applications.
More Information on Silverlight 4 Tools for Microsoft Visual Studio 2010
You can get more information at the Microsoft Silverlight Get Started page.