Generally Visual Studio 2010 and Visual Web Developer 2010 doesn't come with Silverlight 4 out of the box. They have Silverlight 3 capability only.
If you want to develop Silverlight 4 app in any of the above two IDEs, you need to install additonal add-on that can be installed from http://www.microsoft.com/downloads/en/details.aspx?FamilyID=b3deb194-ca86-4fb6-a716-b67c2604a139
Once you have installed, you should start developing Silverlight 4 app. To check whether Silverlight 4 is installed, try to create a new Silverlight project from File menu and click OK, the 2nd dialogue box will have Options dropdown that will let you select the Silverlight version.
In case you are interested to know what's new in Silverlight 4, read this http://www.silverlight.net/getstarted/silverlight-4/
Hope this will help!