5 plants. Lay them out in the pattern like the 5 on a dice. Do not run more than this in a 4 x 4. Do 5 & grow them right & you will do better than doing 9 just mediocre. I have tried it. Very hard to control environment in a space that small with 9 or more plants. They throw off a lot of heat & humidity. You will get better bud by doing just 5, & yield will not be off by much (my yields are higher when I run fewer plants. Use the "mainlining" technique.
You also want to group them as close to the center as you can...as many plants in the sweet spot as possible. As they grow you will have to separate them more. But at any given point, you want them as close to the sweet spot as possible. Some people like to rotate the plants in & out of the sweet spot.
Unless you want to grow sog style, I would take just 5 & do a scrog. Top 3-4 times per plant, LST & mainline & you will have plenty to fill out a 4x4. However, once you fill out the scrog, you can no longer rotate the plants around.
Scrogs suck to work around (in my opinion) but for yield & quality in a small space I think it's the way to go.
