<div>
  <a href="#" class="btn4">
    ボタンはこちら4
  </a>
</div>
.btn4 {
  display: inline-block;
  text-decoration: none;
  background: #3F51B5;
  color: #FFF;
  width: 150px;
  height: 150px;
  line-height: 150px;
  border-radius: 50%;
  text-align: center;
}
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