IF you're house is completely air-tite then outside air is allowed [inside] for the system to function properly. Forced hot air means cooler air is sucked into the heat exchange medium, heated and distributed throughout the dwelling. Eventually the dwelling becomes saturated with the heated air. Doors open windows open so this heat escapes and make up air is needed. That's why outside air is introduced into the system. 4 inch imo is small - 8 inch would be better.