title: Postgresql: inserting spaces between certain type of digits
body: <p>Related to my previous posting
  <a href="https://stackoverflow.com/questions/46234980/postgresql-removing-spaces-between-certain-type-of-digits">Postgresql: removing spaces between certain type of digits</a>,
  I want to do the opposite, namely to insert spaces between postal codes.
  I might have a column with an address such as <code>'01031970 São Paulo SP, BR'</code>.</p>
  
  <p>I want to insert a space between the fourth and fifth numeric digit, i.e.
  <code>'0103 1970 São Paulo SP, BR'</code>. </p>
  
  <p>Any ideas how to do it with <code>regexp_replace</code>?</p>