I was interested by this that came over my desk today:
So, bootstrap is built by a guy, "I have no idea what I am doing"
Alpha 6 out, next the first beta: http://blog.getbootstrap.com/2017/01/06/bootstrap-4-alpha-6/
Random post from Mark in 2014: http://markdotto.com/2014/04/07/dream-on/
All very interesting. People in another post are suggesting September for the final release of 4.0, but I don't think this is official.
Roll on 4.0!! Then we may see Boost able to better use some of the BS4 power.