IMHO
I run a 4x4 myself. A 6 inch extractor is a bare minimum. Ive been tempted lately to get an 8inch even. Its not the cooling i want. Its constantly replacing the air, without letting the air inside the tent sit still that im after. Ive never had mold issues, but i think the quality of my environment would be significantly better. I think yield, smell, and taste would improve.
If you have fresh air coming into the room your tent is in. Id just keep the room dark, and leave a few vents open on your tent.
A good rule of thumb, is to have 3x the surface area for intake, than outtake.
I dont run LED. Though i have, my carbon filter, which connects to a fan. The fan then connects to my cooltube. The cooltube connects to another fan on top, outside my tent, blowing the air straight out.
I always regret, not buying the biggest inline extractor fan i could, with a speed controller.
Definitely gonna be my next purchase, when my current fans have had it.
I put ventilation as the second most important thing, next to light.
I wouldnt skimp on your ventilation at all.