adding glyphicons to bootstrap 4 code example
Example 1: bootstrap glyphicons
<link rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.0.0/css/bootstrap-glyphicons.css">
Example 2: glyphicon cdn bootstrap 4
<link href="//maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" type="text/css" rel="stylesheet">