bootstrap4.css
.modal-dialog-centered {
  display: -ms-flexbox;
  display: flex;
  -ms-flex-align: center;
  align-items: center;
  min-height: calc(100% - 1rem);
}
bootstrap4.css
.modal-dialog-centered::before {
  display: block;
  height: calc(100vh - 1rem);
  content: "";
}
bootstrap4.css
.modal-dialog-centered.modal-dialog-scrollable {
  -ms-flex-direction: column;
  flex-direction: column;
  -ms-flex-pack: center;
  justify-content: center;
  height: 100%;
}
bootstrap4.css
.modal-dialog-centered.modal-dialog-scrollable .modal-content {
  max-height: none;
}
bootstrap4.css
.modal-dialog-centered.modal-dialog-scrollable::before {
  content: none;
}
bootstrap4.css
@media (min-width: 576px) {
  .modal-dialog {
    max-width: 500px;
    margin: 1.75rem auto;
  }
  .modal-dialog-scrollable {
    max-height: calc(100% - 3.5rem);
  }
  .modal-dialog-scrollable .modal-content {
    max-height: calc(100vh - 3.5rem);
  }
  .modal-dialog-centered {
    min-height: calc(100% - 3.5rem);
  }
  .modal-dialog-centered::before {
    height: calc(100vh - 3.5rem);
  }
  .modal-sm {
    max-width: 300px;
  }
}
■モーダル:modal:垂直方向中央に配置するモーダル

  <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>
            <!-- モーダルの×ボタン -->
            <button type="button" class="close" data-dismiss="modal" aria-label="Close">
             <span aria-hidden="true">×</span>
            </button>
          </div>
          <div class="modal-body">
            モーダルの本文が入ります。
          </div>
          <div class="modal-footer">
            <button type="button" class="btn btn-secondary" data-dismiss="modal">閉じる</button>
          </div>
        </div>
      </div>
    </div>
  </div>
  
<style>
.row>.col, .row>[class^=col-] {
  padding-top: 0.5rem;
  padding-bottom: 0.5rem;
  background-color: rgba(0, 0, 0, .1);
  border: 1px solid #aaa;
}
</style>
■モーダル:modal:グリッドレイアウトを使用したモーダル

<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>