title: Oracle insert character into a string
body: <p>I have this table of rows</p>
  
  <pre><code>RowA
  ______
  ABC123
  DEF432
  WER677
  JKL342
  </code></pre>
  
  <p>how can I add a '_' in between the record using oracle to this? 
  Assuming to add on the last 4 character.</p>
  
  <pre><code>RowA
  ______
  ABC_123
  DEF_432
  WER_677
  JKL_342
  </code></pre>