Garage Vent Intake/Exhast
You probably don't need to add the fourth vent. If the fan is rated for 100cfm then 4 inlets will pull roughly 25cfm, 3 about 33cfm and so on. As long as the ducting for the 3 vents are of sufficient size and length to handle 33 cfm it should be just fine. I don't have my chart with me at the moment but a 4" pipe pulling 33 cfm should be good up to 25'. A larger outlet pipe will cut back on noise.