Ventilation - Round Air Vent 4in
CG Series Adjustable Plastic Supply/Exhaust Contour Grille w/ Metal Mounting Collar for 4" Duct
http://www.supplyhouse.com/Fantech-CG4-CG-Series-Adjustable-Plastic-Supply-Exhaust-Contour-Grille-w-Metal-Mounting-Collar-for-4-Duct $12.49 - $11.99
These are adjustable so you can control how much air goes to a particular room. This means that if a room is not being used you can close up the vent for that room so the fresh air mainly goes to the rooms you are using.