That's just at the moment there are 2 current version of Bootstrap 3 and Bootstrap 4.
The English version of this article bootstrap 3 vs 4.
Both versions provide a powerful CSS framework for creating responsive pages, which includes: -multi-column adaptive grid, the elements similar to the navigation buttons, dropdowns etc; -a set of utility classes to control the behavior of elements; -reset or normalize; -configure the printing template; -mediaobject; -some JS-plugins (Carousel, Modal and so on); -and much more.
The first thing that catches the eye is that the source code 4 Bootstrap is written in Less and Sass.
It is also worth noting that the grid Bootstrap 3 was built only to float Ah, while Bootstap 4 invites us to choose between float....and Flexbox!
Change vertical and horizontal alignment on different devices using service tools and classes, such as align-item-center, align-items md-center align-items-lg-start and so on.