Bootstrap 4 comes with custom form elements, To create a custom select menu, add the .custom-select class to <select>
#example.html
<div class="input-group mb-3">
<div class="input-group-prepend">
<label class="input-group-text" for="inputGroupSelect01">Options</label>
</div>
<select class="custom-select" id="inputGroupSelect01">
<option selected>Choose...</option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>
</div>
<div class="input-group mb-3">
<select class="custom-select" id="inputGroupSelect02">
<option selected>Choose...</option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>
<div class="input-group-append">
<label class="input-group-text" for="inputGroupSelect02">Options</label>
</div>
</div>
<div class="input-group mb-3">
<div class="input-group-prepend">
<button class="btn btn-outline-secondary" type="button">Button</button>
</div>
<select class="custom-select" id="inputGroupSelect03">
<option selected>Choose...</option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>
</div>
<div class="input-group">
<select class="custom-select" id="inputGroupSelect04">
<option selected>Choose...</option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>
<div class="input-group-append">
<button class="btn btn-outline-secondary" type="button">Button</button>
</div>
</div>