What is the average rating of viewers 18 to 49 where the total viewer count is 3.93 million and share less than 4?
CREATE TABLE "weekly_ratings" (
  "episode" text,
  "air_date" text,
  "timeslot_est" text,
  "season" real,
  "rating" real,
  "share" real,
  "18_49" real,
  "viewers_m" real,
  "rank_num" text
);
SELECT AVG("18_49") FROM "weekly_ratings" WHERE "viewers_m"=3.93 AND "share"<4;