That is no problem at all! Catel is designed to work with .NET framework 4.0 as well. However, we didn't want to exlcude the 3.5 SP1 users (some companies are not ready to upgrade to 4.0 yet). Simply reference the 3.5 libraries of Catel and all should work
The only additional library that *might* be required is the WPFToolkit.
If people really want a 4.0 version, you can create an issue and we might consider creating a 4.0 library of both the Core and Windows project with the same files.