This is similar to already created thread here: http://stackoverflow.com/questions/3179028/mixed-mode-assembly-in-net-4
Using the app config, I was able to force the assemblies to run on .NET 4. On an XP Machine, I installed just the .NET 4 (without .NET 3.5 or 2.0) and tried to run the built application. It fails to load the mixed mode assembly built in 3.5 framework in .NET 4 without the .NET 3.5 framework on the machine.
Why should it depend on .NET 3.5 when I am forcing the application to run on .NET 4 using App config?