1) Which version of Bootstrap does KendoUI 2018 R2 use?
Technically speaking, the answer is "none". Bootstrap is a third party framework that we do not distribute in our bundles. We do use it as a sort of visual guidelines for the less based Bootstrap theme and we do have a sample project showing that both KendoUI and Bootstrap can co-exist in the same project without interfering with each other.
We do have a sass based kendo-theme-bootstrap that depends on Bootstrap v4 (as in npm dev dependency) and we reuse the variables for homogeneous looks. However, we ship this theme compiled down to css without the dependency. Still, the theme is available at github / npm, should you choose to customize the theme using sass and variables.
So to sum up: while we do not use any specific version of Bootstrap, we do rely on v3 for visual guidelines regarding less based Bootstrap theme and we npm depend on Bootstrap v4 for kendo-theme-bootstrap.
2) Should you upgrade to using Bootstrap v4?
The answer is up to you.
Bootstrap 3 has been around for the longest time and most of its quirks are known. Bootstrap v4 is quite new in that respect and potentially has issues we haven't found yet. Perhaps the biggest pro / con for upgrading or not is the browser support. Bootstrap 4 uses flexbox for the layout part and that raises the support bar to IE11 and up. If you need to support IE version prior 11, you will have hard time using Bootstrap 4. If you don't have to support them, you can safely upgrade.
3) Any other guidelines?
We don't have any specific guidelines. We try to play nicely with Bootstrap 4 so things should integrate well. If there is a bug / incompatibility, we try to fix it in a reasonable timeframe.
My only advice is, should you choose to use Bootstrap 4, is to use a specific version of it, or if you npm depend, again, use a specific version and not ^ version. That's not to say you will not upgrade, but that way you will choose your time to upgrade.
And as usual, if you find any issues with Bootstrap v4 and kendo-theme-bootstrap, do write us back.