what is the highest platform number when the frequency (per hour) is 4, the operator is london overground and the destination is west croydon?
CREATE TABLE "services" (
  "platform" real,
  "frequency_per_hour" real,
  "destination" text,
  "operator" text,
  "line" text
);
SELECT MAX("platform") FROM "services" WHERE "frequency_per_hour"=4 AND "operator"='london overground' AND "destination"='west croydon';