Bootstrap 4 マルチセレクトドロップダウン

$('select').selectpicker();

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap.min.css">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.8.1/css/bootstrap-select.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.8.1/js/bootstrap-select.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.2.0/js/bootstrap.min.js"></script>

<select class="selectpicker" multiple data-live-search="true">
  <option>Mustard</option>
  <option>Ketchup</option>
  <option>Relish</option>
</select>

$('select').selectpicker();

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css">
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.bundle.min.js"></script>

<select class="selectpicker" multiple data-live-search="true">
  <option>Mustard</option>
  <option>Ketchup</option>
  <option>Relish</option>
</select>

$('select').selectpicker();

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.13.1/css/bootstrap-select.css" />
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.bundle.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.13.1/js/bootstrap-select.min.js"></script>

<select class="selectpicker" multiple data-live-search="true">
  <option>Mustard</option>
  <option>Ketchup</option>
  <option>Relish</option>
</select>