MVVM, WPF Ribbon V4, with Prism
Not sure whether we have many Prism specialists on Code Review and I haven't worked with it for a while, so maybe my comments are incorrect at points.  


I do not really like the way you're handling region.Views.CollectionChanged event. This event provides a lot of information in eventArgs but you're ignoring it. Firstly you do not handle removing views at all. Secondly instead of iterating through all views every time I would use those from eventArgs (NewItems property).  
I believe generally it is better to have views isolated from regions in terms of concrete types. Your view is not isolated since it has to inherit RibbonTab class. Maybe it is not an issue here and I'm not sure whether it should be changed somehow, it just looks strange to me.