user: Question: what is the highest platform number when the frequency (per hour) is 4, the operator is London Overground and the destination is West Croydon?
Columns: ['Platform', 'Frequency (per hour)', 'Destination', 'Operator', 'Line']
Types: ['real', 'real', 'text', 'text', 'text']
Sample row: [1.0, 4.0, 'Highbury & Islington', 'London Overground', 'East London']
SQL:

assistant: SELECT MAX("Platform") FROM "Table" WHERE "Frequency (per hour)" = 4.0 AND "Operator" = 'London Overground' AND "Destination" = 'West Croydon';