Each Antminer S9 is going to pull about 300 cfm. So (25) of them implies the need for 7500 cfm both in and out.
Ideally, you could use the fans themselves to push the hot air outside, presuming they had sufficient square footage of ventilation space to achieve that without building up any back pressure.
As a general design goal: you want to keep air moving below 500 feet per minute - any faster and things like leaves will stick to your intake filters.
Do the math, or use an online calculator like
https://www.easycalculation.com/unit-conversion/fpm-to-cfm-calculator.php.
For 7500 CFM at 500 fpm, you need 15 square feet of zero back pressure area to ventilate. You have 4 square feet planned. You will either need to buy intake and exhaust fans that will push 7500 cfm each, or make a bigger hole, or blow power on some serious AC, or overheat your miners.
Remember that filters create back pressure, so you will need to upscale those to adjust. I'd suggest sticking with low MERV filters, like something in the 3-4 range. Those are good enough.
Sorry for the bad news, but its just math, not personal.