title: Filter column with pandas to only show certain string locations
body: <p>I have a dataframe that has a variety of strings in it I need to filter which I am not sure how to filter.</p>
  <pre><code>    locationcode
  0   T4604760374N
  1   T4604760374N
  2   T4604760374N
  3   T4604760374N
  </code></pre>
  <p>I want to make only character 4 to 7 show in this column - what is the best way to do this in pandas?</p>
  <p>I would use substring usually with SQL.</p>
  <p>Thanks!</p>