use glyphicons in 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">
Example 3: glyphicon-user bootstrap 4
<button type="button" class="btn btn-default navbar-btn">Sign in</button>
Example 4: glyphicons in bootstrap 4
<div class="panel panel-primary">...</div>
<div class="panel panel-success">...</div>
<div class="panel panel-info">...</div>
<div class="panel panel-warning">...</div>
<div class="panel panel-danger">...</div>