I have installed Mono 2.10.8 (that should have .NET 4 support) and MonoDevelop 2.8.6.4 on Mac OS X 10.7.3.
I created a new project, but now the IDE displays this error: Error: Framework 'Mono / .NET 4.0' not installed. (Project name).
What do I need to do in order to be able to compile the project?
As suggested, I post this as answer:
MonoDevelop has to be restarted after it updated itself to work with the new .NET 4 features.