title: Remove NA and sort rows by column's values
body: <p>I need to do the 2-steps analysis of the following data:</p>
  
  <pre><code>1 5 1 -2
  2 6 3 4
  1 5 4 -3
  NA NA NA NA
  2 5 4 -4
  </code></pre>
  
  <p>Step 1. Remove all NA rows (these are always whole rows, not cells)
  Step 2. Sort rows by the values of 4th column in descending order</p>
  
  <p>The result should be the following:</p>
  
  <pre><code>2 6 3 4
  1 5 1 -2
  1 5 4 -3
  2 5 4 -4
  </code></pre>
  
  <p>How can I efficiently do this processing, while considering that the data set might be large (e.g. 100,000 entries).</p>