title: Union in more than 2 pandas dataframe
body: <p>I am trying to convert a sql query to python. The sql statement is as follows:</p>
  
  <pre><code>select * from table 1 
  union
  select * from table 2
  union 
  select * from table 3
  union
  select * from table 4
  </code></pre>
  
  <p>Now I have those tables in 4 dataframe <code>df1, df2, df3, df4</code> and I would like to union 4 pandas dataframe which would match the result as the same as sql query.
  I am confused of what operation to be used which is equivalent to sql union?
  Thanks in advance!!</p>
  
  <p>Note:
  The column name for all the dataframes are the same.</p>