This does not answer your question, but for the record -
For instance no responders here to @ganas Migrating to Bootstrap 4
And the pending release runs this
frappe@ubuntu:~/frappe-bench$ find . -name bootstrap.css | xargs grep Bootstrap
* Bootstrap v3.3.1 (http://getbootstrap.com)
frappe@ubuntu:~/frappe-bench$ bench version
erpnext 11.0.3-beta.2
frappe 11.0.0-beta
However now with the Cypress test framework recently introduced, that would help and encourage your upgrade to Bootstrap 4 and the effort too for GUI tests to be added?