9 plants is just too many in that small a space im assuming your planning on going 3x3 and its gonna pack that room by the end of flowering unless you veg em for about 2 weeks if your vegging them 3 weeks you can maybe squeeze 8 in, if you veg 4 weeks you maybe get 6 in there you gotta remember 4x4 is a small space, when i build my room it will be 7x4x8 and i consider it a small flowering room and ill be putting 7 plants in there this run with my autoflower going in the 2x2 veg room, and possibly 10 in the flower room next time trying to create a perpetual harvest but bottom line it all depends on your veg time but if you veg for 4 weeks you need a 5 gallon bucket