I'm running in a 3x3, just two plants, if you count canopy tops, They each have approx 20-25 tops, each plant. If you count main stems then its only 8 per plant.
I'm day 16 atm and I'm worried about running out of room as I'm near capacity on the canopy and in a couple days (day 21) I'm going to defoilate under growth and respread the tops out.... I'm pretty sure it will be wall to wall a week later.
You're thinking you are going to fit 3x what I've got in a 4x4 I just can't see that working out.