What is 4 Season, when 2 Season is "3 168 673 (2 April 2008)"?
CREATE TABLE "rating_figures" (
  "episode" text,
  "1_season" text,
  "2_season" text,
  "3_season" text,
  "4_season" text,
  "5_season" text,
  "6_season" text
);
SELECT "4_season" FROM "rating_figures" WHERE "2_season"='3 168 673 (2 april 2008)';