("Other factors" 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 for 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.
To get the stylesheets to compile, I made a "bridge file" to map old variables in existing-theme Less files to new Sass equivalents or else just to some value, and kept fixing missing variable definitions until the compile errors stopped.
PhpStorm has a Sass file watcher plugin, but you have to get and install the Sass watcher if you don't have it already. It's a Ruby gem. There are also some PHP scripts for compiling Sass files, so we'll have another chance to work the kinks out of that method.
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.
Looks like there are basically two kinds of changes we'll need to cope with. One is the switch from Less to Sass. That's pretty straightforward as it mostly involves file names and variable names in those files (@variable to $variable, etc.). Of course, there are some method differences but probably not many that will cause trouble for us (one I came across is a color function - Less uses "spin" and Sass uses "adjust_color"). Also, paths like the image path variable have slightly different syntax. But I adjusted these as I came across them and will post some docs.
The second kind of change is with the Bootstrap bits themselves. In my "trunk - bootstrap 4 alpha", I found the grid layout and typography worked nicely already (BS4 switches to rem
for the grid and for the typography), but panels and wells have been replaced by the card
class, so in the templates we're going to have to do those replacements. Forms, especially horizontal forms, have changed quite a bit and will need to be adjusted. Also, the navbar is being simplified, as apparently we're not alone in having trouble wrangling content into that area. Menus and tabs weren't working yet either in my installation or at the Bootstrap 4 demo so apparently, they're still being worked out.
Looks like there are also Sass-variable files for Chosen; not sure about Table-sorter, but with Sass being more popular actually than Less, chances are good we can find support.
Interestingly, for Bootstrap 6, they're planning to move from Sass to PostCSS. I'm still reading about PostCSS, which has a lot of momentum. Apparently, its power comes from the modules you add to it; it can do the standard Sass thing but can be configured to do custom things so you're not limited to the abilities of Sass or Less or whatever.
https://github.com/twbs/bootstrap/milestones
http://v4-alpha.getbootstrap.com/
https://scotch.io/bar-talk/whats-new-in-bootstrap-4
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/