The first demo on the B4 documentation for NavBar shows a typical NavBar with branding, page links left justified and then buttons right justified and it works perfectly on mobile. This is one of the standout features of Bootstrap IMHO. This implemented in Foundry would be killer.
Foundry's Navigation Bar is not built using the Bootstrap navigation. This is due to the fact that when I started Foundry in BS4-alpha-1, the navigation bar in Bootstrap did not work -- at all. They'd not even come close to finishing the code for the navigation in BS4 (and they still haven't).
I'm happy to see everyone enthusiastic about this feature. As I've mentioned, it has been noted. I've got some other addons in the works for Foundry right now, so when those are completed and things settle down I will look at this, along with other elements I have planned for Foundry.