title: pandas merge columns in same dataframe
body: <p>I have dataframe with 4 columns. </p>
  
  <pre><code>    Column1    Column2    Column3    Column4
  0   Item1      Value1     Item2      Value2
  1   Item3      Value3     Item4      Value4
  2   Item5      Value5     Item6      Value6
  3   Item7      Value7     Item8      Value8
  4   Item9      Value9     Item10     Value10
  5   Item11     Value11    Item12     Value12
  6   Item13     Value13    Item14     Value14
  </code></pre>
  
  <p>Is there a way for me to combine <code>Column1</code> and <code>Column3</code> together? and with <code>Column2</code> and <code>Column4</code>? To get the below</p>
  
  <pre><code>    Column1    Column2
  0   Item1      Value1
  1   Item2      Value2
  2   Item3      Value3
  3   Item4      Value4
  4   Item5      Value5
  5   Item6      Value6
  6   Item7      Value7
  7   ...
  </code></pre>
  
  <p>I've tried playing with <code>append</code>, <code>concat</code> and <code>split</code> but cant seem to work it out.. </p>