Hi,
You do need both .NET 3.5 SP1 and 4.0 installed. In the mojoportal.sln only the Web UI projects are converted to target 4.0, business layer and data layer projects still target 3.5 but can run under .NET 4.
We need to keep those projects targeting 3.5 so we can still easily produce builds for 3.5 .NET. It would be a major pain to have to make separate projects for both 3.5 and 4.0 for all the projects in the solution because each time we add new files we must do it in each version of the project.
http://www.mojoportal.com/the-net-4-transition-plan-for-mojoportal.aspx
Hope it helps,
Joe