its not about how many fans you have on your exhaust its about having the right CFL for cubic air space in your unit. for a box so small you may not even need an intake fan but just an adjustable vent, naturally this depends on the ambient temperature where the unit is located.
The smallest unit i have is 2 x 2 x 3 ft , which does have an intake and an exhaust 4inch inline fan which are sold as bathroom extractor inline fans, rated around 93CFM . the intake fan only kicks in if the temperature rises to a certasin degree, and cuts back off when it falls to a predefined level. the exhaust i run on a timer to allow humidity to build which is ideal for vegetation. flowering again, also benefits from humidity contruary to popular belief but of course this exposes you to budrot and other funguses... good if you keep a close eye on it, however many growers prefer the safer approach of removing dampness from the air.