CREATE TABLE table_4166 ("No. in series" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" TEXT, "U.S. viewers  ( million ) " TEXT)

What is the production code of the episode that was watched by 3.81 million U.S. viewers?
SELECT "Production code" FROM table_4166 WHERE "U.S. viewers (million)" = '3.81'
SELECT "Production code" FROM "table_4166" WHERE "U.S. viewers (million)" = '3.81'