If you've got VS2010
installed, you ought to find a .NET 4.0 gacutil at
C:\Program Files\Microsoft SDKs\Windows\v7.0A\bin\NETFX 4.0 Tools
The 7.0A Windows SDK
should have been installed alongside VS2010 - 6.0A
will have been installed with VS2008
, and hence won't have .NET 4.0 support.