As it says:

https://www.joomla.org/announcements/release-news/5833-joomla-4-0-0-beta7-and-joomla-3-10-alpha5.html

And Joomla 4 will be shipped with Bootstrap 5 !

https://developer.joomla.org/news/839-joomla-4-0-will-ship-with-bootstrap-5.html