What was the yield of the K-4 explosion?
CREATE TABLE "list_of_artificial_radiation_belts" (
  "explosion" text,
  "location" text,
  "date" text,
  "yield_approximate" text,
  "altitude_km" text,
  "nation_of_origin" text
);
SELECT "yield_approximate" FROM "list_of_artificial_radiation_belts" WHERE "explosion"='K-4';