What is "bootstrap-4"

Bootstrap 4 brought significant changes compared to its predecessor 3. Among the changes we can mention:

Migration from LESS to Sass.
No longer supporting IE8, IE9, and iOS 6.
Added support flexbox and then failed to provide support at the non-flexbox.
Overall increase in font size from 14px to 16px.
Rewrote almost all components, jQuery, plugins and documentation.