Bootstrap 4 is the latest version of Bootstrap which is the most prominent framework for building responsive, mobile-first sites. Integrating with Joomla 4, Bootstrap 4 framework really hit the mark in optimizing the new Joomla 4's features.
You can easily see the big difference between Joomla 3 and Joomla 4 is the front end interface. It is much improved and created a more friendly framework for Joomla users.
Click to this article to read more other features of Bootstraps 4.