We have not tested it but the answer is 'it depends on what you need'.
For example, most (or all) of the data access should work with .NET 4.0.
However, authentication uses Microsoft SimpleMembership, so that will certainly not work.
Also, the new Web API is 4.5 only, so our REST implementation will not work in .NET 4.0.
Hope this helps.
Jack Poorte, Sep 08, 2013