I'm blocked by the requirements even though I fulfill everyone. I had almost the same issue as the referenced here: Cannot install silverlight 4 tools on visual studio 2010 sp1 The difference is that the solution did not work for me and I already did have Visual Web Developer installed.
I tried a complete re-install of VS 2010 and then installed SP1 which should have installed Silverlight4_Tools for me but references to System.Windows.Controls.Input.Toolkit did not work and nothing was added under "...Microsoft SDKs\Silverlight\v4.0". All I see when I look at all the .Net references available are the SL4 library references. For instance, I see "..\Client\System.Windows.Controls.Input.dll" but not "..\Client\System.Windows.Controls.Input.Toolkit.dll" and everything is labeled as version 2.0.5.0 with Runtime V2.0.50727 - which is a .Net 2 framework version and seems strange. The .Web project shows .Net 4 framework on project properties and the SL project show Silverlight 4 (in case anyone is wondering).
Any suggestions of what to try?