In twitter-bootstrap 4 the support for
IE8, IE9, and iOS 6 has been dropped. v4 is now only
IE10+ and iOS 7+.
For sites needing either of those, use v3.
In twitter-bootstrap 4 the official support for
Android v5.0 Lollipop’s Browser and WebView has been Added. Earlier versions of the Android Browser and WebView remain only