PostgreSQL, troca dados de determinada coluna em duas linhas

UPDATE tbl dst
SET doc_num = src.doc_num
FROM tbl src
WHERE dst.id IN(2,4)
AND src.id IN(2,4)
AND dst.id <> src.id -- don't try this at home!
        ;

SELECT * FROm tbl
ORDER BY id;

 id | doc_num |      doc_text      
----+---------+--------------------
  1 |       1 | First column text1
  2 |       3 | First column text2
  4 |       2 | First column text3
  7 |       4 | First column text4
(4 rows)