col-md-4 ml-auto

  <div class="container">
    <button type="button" class="btn btn-secondary" data-toggle="modal" data-target="#exampleModal">モーダルボタン</button>
    <div class="modal fade" id="exampleModal" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel"
      aria-hidden="true">
      <div class="modal-dialog modal-dialog-centered" role="document">
        <div class="modal-content">
          <div class="modal-header">
            <h5 class="modal-title" id="exampleModalLabel">モーダルのタイトル</h5>
            <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span
                aria-hidden="true">×</span></button>
          </div>
          <div class="modal-body">
            <div class="container-fluid">
              <div class="row">
                <div class="col-md-4">col-md-4</div>
                <div class="col-md-4 ml-auto">col-md-4、ml-auto</div>
              </div>
              <div class="row">
                <div class="col-md-3 ml-auto">col-md-3、ml-auto</div>
                <div class="col-md-2 ml-auto">col-md-2、ml-auto</div>
              </div>
            </div>
          </div>
          <div class="modal-footer">
            <button type="button" class="btn btn-secondary" data-dismiss="modal">閉じる</button>
          </div>
        </div>
      </div>
    </div>
  </div>