4x4 will give you exponentially more room and the height will help too.
400 is pretty low for 16 sq ft, but if you keep the plants more in the center with some breathing room around the perimeter it should help. Next run you could upgrade to a 600 or 1000 if you want to.
I know it's not exactly the same, but for reference with more mature plants, this is what my 3x3x6 looks like with 2 flowering plants and 2 tiny plants (cuttings of the bigger ones) just starting to flower:
It fills up quick once they start to stretch and flower!
Running a variety of strains presents its own problems too - my headband plant is about 1.5ft shorter than the blueberry even they started at the same time, it just stretched less. I have a milk crate and crap under it to help balance the canopy. With a bunch of plants, you can keep the shorter strains in the center directly under the light and that way you can get the best coverage without burning them.