Need a bootstrap 3.4.1 to bootstrap 4 transformation

I can't figure out how to translate this from Bs3 to Bootstrap 4:

<!--Site-->
<div class="form-group">
      <div class="input-group">
      <span class="input-group-addon">Site</span>
        <input type="search" name="searchBy" id="searchBy" class="form-control" placeholder="Suchbegriff eingeben"/>
        <div class="input-group-btn">
          <div class="btn-group">
            <button class="btn btn-default dropdown-toggle dropdown-button" type="button" data-toggle="dropdown">
              <span>Search By</span> <span class="caret"></span>
            </button>
            <ul class="dropdown-menu" role="menu">
              <li><a href="#"><</a></li>
              <li><a href="#">></a></li>
            </ul>
          </div>
        </div>
      </div>
    </div>

I tried different ways, but all the time it looks so weird in bootstrap 4 oder even 5. Can u please help me with that?