A major new feature of the .NET Framework 4 is that it can be loaded "in-process" with the .NET Framework 3.5, .NET Framework 3.0, and .NET Framework 2.0 (all of
which use the v2 CLR).
Therefore, to answer your question: Office can now load add-ins that target .NET Framework 4 and add-ins that target .NET Framework 3.5 simultaneously.
For more information about in-process side-by-side execution, see
For more FAQ about Visual Studio Tools for Office, please see
Visual Studio Tools for Office FAQ
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