bootstrap4.css
.btn-group,
.btn-group-vertical {
  position: relative;
  display: -ms-inline-flexbox;
  display: inline-flex;
  vertical-align: middle;
}
bootstrap4.css
.btn-group > .btn,
.btn-group-vertical > .btn {
  position: relative;
  -ms-flex: 1 1 auto;
  flex: 1 1 auto;
}
bootstrap4.css
.btn-group > .btn:hover,
.btn-group-vertical > .btn:hover {
  z-index: 1;
}
bootstrap4.css
.btn-group > .btn:focus, .btn-group > .btn:active, .btn-group > .btn.active,
.btn-group-vertical > .btn:focus,
.btn-group-vertical > .btn:active,
.btn-group-vertical > .btn.active {
  z-index: 1;
}
bootstrap4.css
.btn-group-vertical {
  -ms-flex-direction: column;
  flex-direction: column;
  -ms-flex-align: start;
  align-items: flex-start;
  -ms-flex-pack: center;
  justify-content: center;
}
bootstrap4.css
.btn-group-vertical > .btn,
.btn-group-vertical > .btn-group {
  width: 100%;
}
bootstrap4.css
.btn-group-vertical > .btn:not(:first-child),
.btn-group-vertical > .btn-group:not(:first-child) {
  margin-top: -1px;
}
bootstrap4.css
.btn-group-vertical > .btn:not(:last-child):not(.dropdown-toggle),
.btn-group-vertical > .btn-group:not(:last-child) > .btn {
  border-bottom-right-radius: 0;
  border-bottom-left-radius: 0;
}
bootstrap4.css
.btn-group-vertical > .btn:not(:first-child),
.btn-group-vertical > .btn-group:not(:first-child) > .btn {
  border-top-left-radius: 0;
  border-top-right-radius: 0;
}
■ボタン:button:コンポーネント:垂直方向ボタングループ btn-group-vertical

  <div class="btn-group-vertical" role="group" aria-label="ドロップダウンを含むボタングループ">
    <button type="button" class="btn btn-secondary">上ボタン</button>
    <button type="button" class="btn btn-secondary">中ボタン</button>
    <div class="btn-group" role="group">
       <button id="drop2" type="button" class="btn btn-secondary dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
        ドロップダウン
      </button>
      <div class="dropdown-menu" aria-labelledby="drop2">
        <a class="dropdown-item" href="#">リンク1</a>
        <a class="dropdown-item" href="#">リンク2</a>
      </div>
    </div>
    <button type="button" class="btn btn-secondary">下ボタン</button>
  </div>