title: Keep only the 1st line in every 4 lines and delete the other lines
body: <p>I have a file like the following</p>
  
  <pre><code>a
  b
  c
  d
  e
  f
  g
  h
  i
  j
  k
  </code></pre>
  
  <p>and I want it to be:</p>
  
  <pre><code>a
  e
  i
  </code></pre>
  
  <p>How to do it in vim?</p>