On what surface was the game played with a score of  6–4, 6–4?
CREATE TABLE "women_s_doubles_10_4_titles_6_runner_ups" (
  "outcome" text,
  "year" real,
  "championship" text,
  "surface" text,
  "partner" text,
  "opponents" text,
  "score" text
);
SELECT "surface" FROM "women_s_doubles_10_4_titles_6_runner_ups" WHERE "score"='6–4, 6–4';