Before we jump into the component details, let's review the new browser support. If you are currently running on version 3 and support some older browsers, you may need to adjust your support level when migrating to Bootstrap 4. For desktop browsers, Internet Explorer version 8 support has been dropped. The new minimum Internet Explorer version that is supported is version 9.
In terms of mobiles, iOS version 6 support has been dropped. The minimum iOS supported is now version 7. The Bootstrap team has also added support for Android v5.0 Lollipop's browser and WebView. Earlier versions of the Android Browser and WebView are not officially supported by Bootstrap.