Yes your hcv is probably stuck, You can sometimes get them free by turning the heating switch hot to cold until you feel the temperature change. however to lose 4 litres of coolant you must also have a leak somewhere so check the hcv pipes as these go very brittle ,also check the thermostat housing as these also can crack, and also check the header tank cap seal isn't letting by which would cause the car to overheat and lose the coolant that way