https://i.ytimg.com/vi/W9AjhGko2mA/maxresdefault.jpg
The fourth one controls recirculate, and it allowed you to set in-between positions between 100% recirculate and 100% fresh air.
To simultaneously optimize pollution/humidity/CO2 I find I prefer about 80% recirculate and 20% fresh air, but achieving this configuration is impossible in most (all?) modern cars.