After playing with some Silverlight beta bits, going back to RTM, etc. I could not compile F# projects for Silverlight 4 anymore. Even after reinstalling everything in [what I think is] the right sequence I was still getting this error:
F# runtime for Silverlight version v4.0 is not installed.
Please go to http://go.microsoft.com/fwlink/?LinkId=177463 to download and install matching F# runtime
C:\Program Files (x86)\Microsoft F#\v4.0\Microsoft.FSharp.Targets
I followed the link and re-installed the Microsoft Silverlight 4 Tools for Visual Studio 2010 (which I already did), but the error was still there. Looking into c:\program files (x86)\Microsoft F#\Silverlight\Libraries\Client I noticed that the folder v3.0 was there, but not v4.0.
If you have the same issue, go to http://go.microsoft.com/fwlink/?LinkId=177463 and download the Silverlight 4 Tools installer (Silverlight4_Tools.exe).
Don’t run it, instead extract its contents with your zip tool of choice (I used WinRar) and run FSharpRuntimeSL4.msi.
Now look again in c:\program files (x86)\Microsoft F#\Silverlight\Libraries\Client, a v4.0 folder should have appeared. If you have it your Silverlight 4 F# projects should now compile.