What surface was played on with a score of 6–4, 6–3 and on 4 May 1992?
CREATE TABLE "doubles_champion_9" (
  "date" text,
  "tournament" text,
  "surface" text,
  "partnering" text,
  "opponent_in_the_final" text,
  "score" text
);
SELECT "surface" FROM "doubles_champion_9" WHERE "score"='6–4, 6–3' AND "date"='4 may 1992';