HTML

bootstrap 4 selectpicker

<select class="selectpicker">
  <option>Mustard</option>
  <option>Ketchup</option>
  <option>Barbecue</option>
</select>

bootstrap 4 selectpicker

$(function () {
    $('select').selectpicker();
});

bootstrap 4 selectpicker

// To style only selects with the my-select class
$('.my-select').selectpicker();

bootstrap 4 selectpicker

// To style all selects
$('select').selectpicker();

bootstrap 4 selectpicker

<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap-select@1.13.14/dist/css/bootstrap-select.min.css">

<!-- Latest compiled and minified JavaScript -->
<script src="https://cdn.jsdelivr.net/npm/bootstrap-select@1.13.14/dist/js/bootstrap-select.min.js"></script>

<!-- (Optional) Latest compiled and minified JavaScript translation files -->
<script src="https://cdn.jsdelivr.net/npm/bootstrap-select@1.13.14/dist/js/i18n/defaults-*.min.js"></script>