user: Question: Which date has a Terrain of Plain stage, and a Stage of 4?
Columns: ['Stage', 'Date', 'Route', 'Terrain', 'Length', 'Winner']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['P', '28 June', 'Plumelec', 'Individual time trial', 'km (mi)', 'Bernard Hinault ( FRA )']
SQL:

assistant: SELECT "Date" FROM "Table" WHERE "Terrain" = 'Plain stage' AND "Stage" = '4';