Hi Ian and Sadegh,
There's no reason Bootstrap 4 can't be used with mojoPortal. Truth is, we've spent a lot of time on our Framework skin and it includes many of the things (i.e., flexbox) that Bootstrap 4 does (although implemented differently).
For us to implement Bootstrap 4 with Framework would take a bit of time and the end result would be roughly the same capability we have now. With that said, it is important for others to be able to easily use mojoPortal without having to learn how we implemented CSS standards like flexbox in our Framework skin.
There are two short answers to this: 1) Bootstrap 4 can be implemented in mojoPortal; 2) we just haven't gotten around to creating a skin utilizing Bootstrap 4, yet.