I wanna grow in my closet with a 4" carbon filter fan but do i need fresh air blowing in? i will be having the air that is being sucked in from the fan will be going outside the closet, is that god enough or should i have air being pumped in??
depends on the temperature. if you are using a 600 watt hps and its 25 degrees outside then no its not enough if you are using 200 watts of cfls and its cold enough outside you can keep your room at 22 degrees then youre golden. you can try it with just the 4" fan and monitor the temperature, if its getting above 28 degrees then you need to supply fresh air or have a bigger exhaust fan.
Just blowing the spent air outside the closet won't do. You need to get it out of room otherwise it will just be drawn back in, can you vent it out of a window or into the loft/ crawl space? A lot of beginners think my temps are ok, so that means the airflow is fine. It doesn't. Plants need lots of fresh air, far more than most people realize. They also need the air to move around the leaves to prevent a layer of still, spent air building up on them...
Get one bad ass exhaust fan moving 450+ cfm. Then a small opening for air to come in. Vent exhaust into attic or window like he said above ^^. If you dont have fresh air your plants will suffer. Fresh air is very important.
You don't have to have fresh air "blowing" in, but you do need for the grow space to have access to fresh air that can be drawn in. You can't exhaust a room or space that has no source of air to replace it. And besides, the plants need fresh air just like you do.