You don't need 4 plants to utilize all your light, just train your plants into a small SCRoG and force them to grow around the sides of the box, 4 plants is going to be way too crowded for that space.
Also 80 CFM is assuming there's nothing blocking the fan from pulling air, like say a carbon filter. 80CFM refers to the amount of air displaced when the fan is running literally on its own, on a table, in a lab, with normalized air pressure, at sea level, at average temp and humidity.
You want a stronger fan especially if you're using passive instead of forced intakes since 1 80CFM fan is not going to be able to produce enough negative pressure inside of your box to prevent air from leaking out of your passive intakes.
Also, you don't want that 2nd fan pointing down from the top of your box, if your intakes are at the bottom and exhaust is at the top, you're going to be blowing air against the natural flow you're trying to create with a passive intake and low CFM exhaust like this. You want to put the fan at the bottom, and most likely more than 1 so you can get decent airflow to all of your inner foliage to prevent mold and all those fun things that can happen with lack of ventilation.
