Visual Studio 2010 or Visual Web Developer Express 2010 or Visual Phone Developer Express 2010 that matches the language version of Silverlight Tools 4 must be installed before installation of Silverlight Tools can continue.
I was running into the above error when I tried to install Silverlight 4 Tools for Visual Studio 2010 whether I did it from the downloaded install or via the Web Platform Installer.
Downloaded Silverlight4_Tools.exe error message:
WPI:
If you open the first log file you will find the error at the bottom:
Installation Blockers:
1) Visual Studio 2010 or Visual Web Developer Express 2010 or Visual Phone Developer Express 2010 that matches the language version of Silverlight Tools 4 must be installed before installation of Silverlight Tools can continue. Silverlight Tools is available in other languages at <A HREF="http://go.microsoft.com/fwlink/?LinkId=177432">http://go.microsoft.com/fwlink/?LinkId=177432</A>.
When I looked online for this error I was only seeing references to the Beta and Release Candidates of Visual Studio 2010.
After a little thought, I decided to open up the readme file for the SP1 Beta of Visual Studio 2010 that I installed a while back and there was the answer:
2.1.4. Error when you try to install Silverlight 4 Tools for Visual Studio 2010 on Visual Studio 2010 SP1 Beta
An error message may appear when you try to install Silverlight 4 Tools for Visual Studio 2010 on Visual Studio 2010 SP1 Beta.
To resolve this issue:
Because Silverlight 4 Tools for Visual Studio 2010 (http://www.microsoft.com/downloads/en/details.aspx?FamilyID=b3deb194-ca86-4fb6-a716-b67c2604a139&displayLang=en) is included in Visual Studio 2010 SP1 Beta, you do not have to install Silverlight 4 tools separately. Also, you do not have to uninstall Silverlight 4 Tools for Visual Studio 2010 before you install Visual Studio SP1 Beta.
There you have it. If you installed VS 2010 SP1 Beta then you already have the Silverlight 4 Tools. How’s that for a descriptive error message. 
Happy Coding!