What is the first year where the assassination attempt was at Martyrs' Mausoleum, Rangoon, Burma?
CREATE TABLE "table1_251272_4" (
  "year" real,
  "date" text,
  "intended_victim_s" text,
  "title_at_the_time" text,
  "location" text,
  "would_be_assassin_s" text,
  "method" text,
  "initial_outcome_notes" text
);
SELECT MIN("year") FROM "table1_251272_4" WHERE "location"='Martyrs'' Mausoleum, Rangoon, Burma';