what are all the u15 3rd iv with u15 4th iv being bbc
CREATE TABLE "winning_head_of_the_river_championship_e" (
  "crew" real,
  "open_1st_viii" text,
  "open_2nd_viii" text,
  "open_3rd_viii" text,
  "u16_1st_viii" text,
  "u16_2nd_viii" text,
  "u16_3rd_viii" text,
  "u15_1st_iv" text,
  "u15_2nd_iv" text,
  "u15_3rd_iv" text,
  "u15_4th_iv" text,
  "u15_5th_iv" text,
  "u15_6th_iv" text
);
SELECT "u15_3rd_iv" FROM "winning_head_of_the_river_championship_e" WHERE "u15_4th_iv"='BBC';