putting rocks in for drainage is BAD BAD BAD. it doesnt alow for drainage, it raises some kind of water level in the dirt and doesnt allow the roots to go deep enough into the soil to do what they are supposed to do. so when it comes to rocks JUST SAY No!!!

as far as the number of plants for a 4x4 area, I would put 4 please in that area.

where are you that you are allowed to grow outside??