Travelcard Zone 4 is the fourth outward zone of Transport for London's zonal system. The zonal system used for calculating co-ordinated inter-modal Travelcard fares within London. In the Travelcard zonal system, London is split into nine approximately concentric zones for the purpose of determining the cost of a Travelcard. The zone is also used for pricing London Underground and Docklands Light Railway single journeys, but not journeys undertaken entirely by bus or tram, for which a city-wide flat fare applies.