Micro Grow Ventilation Help Please!?!?!

ok so i have a 1.5x2.5x3.2 box im going to do 2 plants cause the filter takes up too much room for a third.. now i have a 4 in 170cfm fan and a 4x12 phat filter, i got the whole thing together and muffled. My question is how big should my input/inputs be. I want to use negative air pressure for the inlets. I just dont know how big to make them. Same as the output at 4 in? double that? just not sure i need it to be quiet as posible while still ensuring that any air in the box still has to leave thru the filter so it does not get smelly in my closet. Thanks.

If you want negitive pressure, then you want more on the exhast side. So like 2 fans on the out, and 1 on the in.

well actually you dont need inlet fans with negative air pressure... thus negative air pressure. I just need inlet sizing.... but thanks for chipping in man