title: Calculating the number of rows that have specific value in one column (Python)
body: <p>I have a dataframe with 4 columns. The 4th column contains numbers from 1-150 and I would like to caluclate the number of rows that have the same value n (from 1-150) in column 4. As a simplified example:</p>
  <pre><code>a = [[1, 2, 3, 1], [4, 3, 4, 1], [2, 7, 8, 1], [3, 3, 2, 2], [2, 3, 3, 3], [4 ,5, 5, 3]]
  
  should return me a list  
  a_new = [3, 1, 2]  
  </code></pre>
  <p>How can I do this?</p>