CREATE TABLE "table1_25721_4" (
  "release" text,
  "num_of_discs" real,
  "region_1" text,
  "region_2" text,
  "region_4" text
);

Question: How many DVDs where were in the complete collection series 1-8 with extras?
SQL: SELECT "num_of_discs" FROM "table1_25721_4" WHERE "release"='The Complete Collection Series 1-8 with extras';

Question: Which release had 6 DVDs?
SQL: SELECT "release" FROM "table1_25721_4" WHERE "num_of_discs"=6;

Question: How many region 1's did back to earth have?
SQL: SELECT COUNT("region_1") FROM "table1_25721_4" WHERE "release"='Back to Earth';