<link href="//maxcdn.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css" rel="stylesheet" id="bootstrap-css">
<script src="//maxcdn.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.min.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<div class="container">
<div class="row">
<ul>
<li><a href="#">First</a></li>
<li class="has-submenu"><a class="link" href="#">Second</a>
<ul class="submenu">
<li><a href="#">submenu 1</a></li>
<li><a href="#">submenu 1</a></li>
</ul>
</li>
<li><a href="#">Third</a></li>
<li class="has-submenu"><a class="link" href="#">Fourth</a>
<ul class="submenu">
<li><a href="#">submenu 2</a></li>
<li><a href="#">submenu 2</a></li>
</ul>
</li>
</ul>
</div>
</div>