yea if i get that inline fan you said, room would exchange like every 15 seconds
That's fine. In a grow box situation I don't think you can go wrong with too much cfm. Creates more negative pressure in the room itself. Easier to keep smell under control and it is kind of like having a fan in the room blowing on the plants. Here I'll try to explain it.
Your grow box you say is 3x3x4.5 I assumed that was in Feet. Anyways, you want negative pressure. Basically you have the fan sucking air out of the box, you have a hole at the bottom of the box. So the air is being sucked in from the bottom and the hot air is going through the fan and out of the box. I don't believe in intake fans because that IMO creates a situation when you have air not flowing throughout the entire box as you would with negative pressure. That's a bit off topic but in essence. If your box ix 3'x3'x4.5' yes I would go with the fan I linked.