Boostrap 4 间距，为什么按钮没有版本 3 中的垂直间距？

版本 3 布局（ https://plnkr.co/edit/XDA9EwMAzH4xDSfZaXvS?p=preview）

<div class="container">
  <h1>Confirm your details</h1>
  <div class="row">
    <div class="col-md-12">
      <div class="panel">
          <h4 class="panel-heading">We need to confirm your current address details and contact information.</h4>
          <p class="panel-body">With supporting text below as a natural lead-in to additional content.</p>
      </div>
    </div>
  </div>
  <div class="row">
    <div class="col-md-12 text-right">
      <p>
        <a href="#" class="btn btn-primary">Confirm</a>
      </p>
    </div>
  </div>
</div>

版本 4 布局（ https://plnkr.co/edit/LDy00Kk6eVHDq85ZqjgQ?p=preview）

<div class="container">
  <h1>Confirm your details</h1>
  <div class="row">
    <div class="col">
      <div class="card">
        <div class="card-block">
          <h4 class="card-title">We need to confirm your current address details and contact information.</h4>
          <p class="card-text">With supporting text below as a natural lead-in to additional content.</p>
        </div>
      </div>
    </div>
  </div>
  <div class="row">
    <div class="col-md-12 text-right">
      <p>
        <a href="#" class="btn btn-primary">Confirm</a>
      </p>
    </div>
  </div>
</div>

为什么按钮没有垂直空间，我应该如何在 Bootstrap v4 中添加它？