CREATE TABLE table_21304155_1 (original_air_date VARCHAR2, us_viewers__millions_ VARCHAR2)

What was the original air date of the episode that had 4.08 million U.S. viewers?
SELECT original_air_date FROM table_21304155_1 WHERE us_viewers__millions_ = "4.08"
SELECT "original_air_date" FROM "table_21304155_1" WHERE "4.08" = "us_viewers__millions_"