"So in a 4x4 that's 12 plants right and 6x6 is 36 plants"
Depends on how you want to grow your plants, you could just let 'em grow, FIM/top them so they bush out, or try Sea of Green. You might want to figure or right a full plan of your grow, it'll save you a lot of headaches. I also agree with the 2 600 watters, its usually better to go with more lower wattage lights, also 600s are the most efficient wattage.