Tiki 19 will move on from Bootstrap 3 to Bootstrap 4, as its CSS framework. I'm in the process of updating the themes at this site to be compatible. As I'm also working on the Tiki project files, progress on everything is slow but sure. Tiki 19 is scheduled to be released in October. There are some cool new features in Bootstrap 4, such as Flexbox layout, and in Tiki, like HP5 support, so the future looks bright!