XamarinStudios 4 is "Open Source" because its just MonoDevelop 4 with a new name.
Just like Googles Chrome is basically Chromium shipped with closed source peaces, it looks like Xamarin is just doing the same thing. This does not mean "XamarinStudios" is a closed source project. Even looking in the about dialog it say's its under the GPL licence.
Here is even a commit to MonoDevelop(aka XamarinStudios) on github calling MonoDevelop XamarinStudios: https://github.com/mono/monodevelop/...d8dfd7181fd64e