Which Date has a Margin of victory of 4 strokes?
CREATE TABLE "senior_pga_tour_wins_11" (
  "date" text,
  "tournament" text,
  "winning_score" text,
  "margin_of_victory" text,
  "runner_s_up" text
);
SELECT "date" FROM "senior_pga_tour_wins_11" WHERE "margin_of_victory"='4 strokes';