I would make the suggestion of going LED since you're doing a scrog. This will eliminate your choice of needing a 4 or 6 inch fan, as you'll only need a 4 inch inline fan to pull air through the scrubber and push it out the box, and a vent hole at the bottom for air intake/negative pressure. This will also give you more headroom since the typical LED panel is about 2-3 inches thick.