Silverlight 4 will not have support for Visual Studio 2008. You must need Visual Studio 2010 to start working on Silverlight 4.
Visual Studio 2010 supports both Silverlight 3 and Silverlight 4. Hence, you can use that for both application development.
Microsoft Released Visual Studio 2010 and you can get the informations on those and also the download links here: Microsoft Visual Studio 2010
Silverlight 4 will be release tomorrow and hence you have to wait for a single day if you have already installed the Visual Studio 2010 (Release) for your Silverlight 4 development.
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