Try the following if you get this error in Visual Studio:
The imported project "C:\Program Files (x86)\MSBuild\Microsoft\Silverlight\v4.0\Microsoft.Silverlight.CSharp.targets" was not found. Confirm that the path in the <Import> declaration is correct, and that the file exists on disk.
1. Uninstall all versions of Silverlight (not sure if this is required, but I was getting issues when I tried to install SilverLight 4)
Add/Remove Programs –> Search for Silverlight—> Uninstall
2. Install the Silverlight 4 SDK
3. Go back to visual studio and reload the project.
That resolved the issue for me.