The monodevelop.com website just takes me to this "Xamarin Studio" thing that has severe restrictions on the usage of the free version, and pricing that's almost as bad as Visual Studio on the paid versions! Is MonoDevelop for Windows dead? All I want to do is develop a GTK# application without installing Linux!
There's used to be a version up on Sourceforge, but 9 years after I wrote this answer it seems to have disappeared. Sounds reasonable enough...
If you really need version 4 you'll have to build it yourself. If you still have/find the sources that is.