What is the total number of total viewers with a share of 18.8% and a weekly ranking under 16?
CREATE TABLE "series_4" (
  "episode_no" real,
  "airdate" text,
  "total_viewers" real,
  "share" text,
  "bbc_one_weekly_ranking" real
);
SELECT COUNT("total_viewers") FROM "series_4" WHERE "share"='18.8%' AND "bbc_one_weekly_ranking"<16;