well not knowing what lights your running, or what else you have in the room that puts off heat. just going off the fact that your using 4 in fans....
I would use a 6 in inline to exhaust, make shure the exhaust fan is pulling air out from the top of your grow area. make sure your intake is somewhere reasonably cool. eg the intake is indoor air from the room, or fresh air from outside, the basement... and the hot exhaust is going somewhere else (outside, another room, whatever just not the same room as the exhaust. hth