There is a Silverlight project which created in Silverlight 2, but when it opened in Visual Studio 2010 which converts the Silverlight 2 project to Silverlight 4, the system.web.silverlight reference can not be located because there is no more system.web.silverlight in Silverlight 3 / 4, which meas you can not find system.web.silverlight.dll file.
The reason is because there is no more system.web.silverlight in Silverlight 3 / 4 which is mentioned above, Silverlight 3 and silverlight 4 no more use <asp:Silverlight> tag for silverlight control but use <object> tag, for example:
<object width="300" height="300"
<param name="source" value="SilverlightApplication1.xap"/>
more info on creating object tag please check MSDN (http://msdn.microsoft.com/en-us/library/cc189089(VS.95).aspx)