CREATE TABLE table_72138 ("No. in season" FLOAT, "No. in series" FLOAT, "Title" TEXT, "Canadian airdate" TEXT, "US airdate" TEXT, "Production code" FLOAT)

The U.S. airdate of 4 april 2008 had a production code of what?
SELECT MAX("Production code") FROM table_72138 WHERE "US airdate" = '4 April 2008'
SELECT MAX("Production code") FROM "table_72138" WHERE "US airdate" = '4 April 2008'