title: regex for four-digit numbers (or "default")
body: <p>I need a regex for four-digit numbers separated by comma ("default" can also be a value).</p>
  
  <p>Examples:</p>
  
  <ol>
  <li><code>6755</code>  </li>
  <li><code>3452,8767,9865,8766,3454</code>  </li>
  <li><code>7678,9876</code>   </li>
  <li><code>1234,9867,6876,9865</code>  </li>
  <li><code>default</code></li>
  </ol>
  
  <p>Note: <code>"default"</code> alone should match, but <code>default,1234,7656</code> should NOT match.</p>