Bootstrap 4 supports the latest stable release of all modern browsers on both desktop and mobile devices, that is, Firefox, Chrome, Opera, Safari, Internet Explorer 10 - 11, and Microsoft Edge. Furthermore:
(source: official Bootstrap 4 documentation)
It is important to note that unlike its predecessor, Bootstrap 4 no longer supports Internet Explorer 9 and earlier versions. The decision to only support newer versions of Internet Explorer was a reasonable one, as Microsoft itself doesn't provide technical support and updates for Internet Explorer 8 (and earlier versions) anymore (as of January 2016). Furthermore, as we will discover later on in this book, Bootstrap 4 relies on flexbox, which is also not supported by older browsers.
Furthermore, Internet Explorer 8 (and earlier) does not support rem, meaning that Bootstrap 4 would have been required to provide a workaround. This, in turn, would most likely have implied a large amount of additional development work, with the potential for inconsistencies. Lastly, responsive website development for Internet Explorer 8 (and earlier) is difficult, as the browser does not support CSS media queries. Given these three factors, dropping support for this version of Internet Explorer was the most sensible path of action.