Name the became consort for marriage 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 "became_consort" FROM "house_of_oldenburg" WHERE "marriage"='4 april 1721';