Bootstrap 4 Ya

Ejemplo
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<!doctype html>
<html>
<head>
  <title>Prueba de Bootstrap 4</title> 
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
  <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
</head>
<body>
  <div class="container">
    <div class="dropdown">
      <button class="btn btn-primary dropdown-toggle" type="button" id="dropdown1" data-toggle="dropdown">
        Buscadores
      </button>
      <div class="dropdown-menu">
        <a class="dropdown-item" href="http://www.google.com">Google</a>
        <a class="dropdown-item" href="http://www.bing.com">Bing</a>
        <a class="dropdown-item" href="http://www.yahoo.com">Yahoo</a>
      </div>
    </div>  
  </div>