Without further ado, open your favorite browser and visit the Migrating to Bootstrap 4 documentation page at https://getbootstrap.com/docs/4.0/migration/:
As you can see on the Migrating to v4 documentation page, there are several things on the left, and at the bottom we have Migration. Here, you will see a lot of the different changes that Bootstrap 4 introduced, of which a few were discussed earlier. It has dropped the support for IE8, IE9, and iOS6, which is only for IE10+ and iOS 7+. It has mobile browser support as well.
In the Global changes section, you will see some of the big changes. Flexbox is enabled by default, so they're getting away from things such as floats; they've switched from LESS to Sass for CSS; and another thing is they're favoring rems instead of px. px is still being used, but there's a lot more move toward rem, and...