Hello jonghun kang,
I am afraid that Silverlight 4 development is not possible with VS 2008. For example, Silverlight 4 Tools is only available for Visual Studio 2010 which is the core Silverlight 4 Dev package.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
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