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