As of this writing, the Bootstrap 4 development is at its sixth alpha release, so eventually there will be a beta release phase, and then the official release. There's no official timetable for this but we do know it's coming so we want to get prepared. Tiki, since version 13, is integrating Bootstrap 3 for a layout and styling framework. When Bootstrap 4.0 is released and other factors are positive, Tiki will shift from Bootstrap 3 to Bootstrap 4.
(By other factors, this might be such things as outside theme developers/designers shifting to Bootstrap 4 as well, other themes being available for Bootstrap 4 and so on. But given the long gestation period, the cool things brought by Bootstrap 4, and more and more third parties releasing Bootstrap 4 items, the transition to it is apt to be fairly fast.)
To check what's necessary in the transition from 3 to 4, I made a new trunk installation on my local server, and put the Bootstrap 4 package in the vendors_custom directory. I chose one theme to test with - Amelia, probably because it's at the top of the list alphabetically. I used PhpStorm and its SCSS plugin for compiling.
In my test, I just modified one Smarty file - templates/layouts/basic/layout_view.tpl - because on first view the page columns were out of order. I rearranged div#col2, div#col1, and div#col3 into their actual order on the page. I haven't researched it yet, but it seems the "pull", "push", and "offset" classes are no longer used. In Bootstrap 4, column layout uses Flexbox by default, so in grids there is no longer floating columns into place, adding clearfix, and so on.
With just the basic adjustments to the Amelia theme files, the basic modification of layout_view.tpl, and pointing to the Bootstrap 4 lib (ignoring the Bootstrap 3 lib), the result was that the Tiki site did look and work pretty much like a Tiki site. But there were some problems:
I understand forms have also undergone significant changes but I didn't look yet at what updating will be needed.
There's new JavaScript for modals and other JS-related changes.
I'll add to this page as I do more testing, etc.
http://v4-alpha.getbootstrap.com/
http://bootstrapbay.com/bootstrap4/ (Bootstrap 4 Cheatsheet)
https://medium.com/wdstack/bootstrap-4-whats-new-visual-guide-c84dd81d8387#.b2sxbfsf6
http://www.hongkiat.com/blog/boostrap-4-best-features/
https://hackerthemes.com/bootstrap-cheatsheet/
https://bootstrapcreative.com/resources/bootstrap-4-css-classes-index/
https://www.udemy.com/bootstrap-4/
http://codepen.io/search?q=bootstrap+cards&limit=all&depth=everything&show_forks=false
https://gist.github.com/chriseppstein/674726 - Sass/Less Comparison
https://css-tricks.com/sass-vs-less/