Selecting the mid range of 115 air changes per hour the total volume per hour need is 460,000 cubic feet per hour (4,000*115). Divide the total volume per hour by 60 will give us 7,667 cubic feet per minute. Using the specified 720 cubic feet per minute per filter we divide 7,667 by 720 we get 10.6 round it up to 11 filters.