human
Considering the provided database schema and associated query, produce SQL code to retrieve the answer to the query. 
###Database Schema: No. in season, No. in series, Title, Canadian airdate, US airdate, Production code
###Question: The U.S. airdate of 4 april 2008 had a production code of what?
gpt
SELECT MAX Production code FROM table WHERE US airdate = 4 April 2008