Name the birth of the person that has a spouse of frederick iv and became consort of 4 april 1721
CREATE TABLE "house_of_oldenburg" (
  "birth" text,
  "marriage" text,
  "became_consort" text,
  "ceased_to_be_consort" text,
  "death" text,
  "spouse" text
);
SELECT "birth" FROM "house_of_oldenburg" WHERE "spouse"='frederick iv' AND "became_consort"='4 april 1721';