The U.S. airdate of 4 april 2008 had a production code of what?
CREATE TABLE table_72138 (
    "No. in season" real,
    "No. in series" real,
    "Title" text,
    "Canadian airdate" text,
    "US airdate" text,
    "Production code" real
)
SELECT MAX("Production code") FROM table_72138 WHERE "US airdate" = '4 April 2008'
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: The U.S. airdate of 4 april 2008 had a production code of what? ### Input: CREATE TABLE table_72138 (
    "No. in season" real,
    "No. in series" real,
    "Title" text,
    "Canadian airdate" text,
    "US airdate" text,
    "Production code" real
) ### Response: SELECT MAX("Production code") FROM table_72138 WHERE "US airdate" = '4 April 2008'