The toolset is not Bootstrap 4 compatible.
I tried personally Bootstrap 4 with Toolset, and while most of the things work, some don't, especially the new features that have been added.
You have basically 2 options:
- run Bootstrap 3.xx loaded by Toolset and add the required code for new elements in custom scripts
- or enqueue Bootstrap 4 thru your theme, and do not load Bootstrap in Toolset (set that the Theme loads it already).
Now, of course, Toolset will assume that Bootstrap 3.xx is loaded by the theme since there is no other option, or you can entirely choose that this site is not using Bootstrap, in either case, everything of Bootstrap 4.xx will now work, but you will need to add several things manually (the HTML structure) as Toolset logically will not be able to produce them.
Hence, for example, Toolset Layout Rows and containers and cells should proceed to work, while for example panels, thumbnails, wells etc will stop work in favour of cards.
You can check up all the differences here:
hidden link
Basically, all that is different from Bootstrap 3.0 needs to be added manually, since Toolset is not able to produce or use it out of the box.
We have a request filed to integrate Toolset with 4.xx versions, and I have added your vote to it.
For now though, above is the only way to "mix" both.
Let me know if it works for you.