title: Converting numbers to particular string format in a pandas DataFrame
body: <p>I need to run a task which can be done with a loop, but I imagine that there is a more efficient and pretty way to do this. I have a <code>DataFrame</code> which has an integer column, which I would like to transform to a 4-digit string representation. That is, 3 should be converted to '0003', 234 should be converted to '0234'. I am looking for a vector operation that will do this to all entries in the column at once, quickly with simple code.</p>