Back to plant size and how much work you're willing to put into it. Are you looking at growing tall, medium, short, or dwarf strains? Bushy Indica or lanky Sativa?
Four autos in 5-gallon fabric pots max my 3x3 with plenty of defoliation and flower maneuvering. If I ever get back to fem, I'll probably wind up with 2 plants getting 3 toppings in 7-gallon unless I want to up my bag and saucer size to 10-gallon.
If I had a 4x4, I wouldn't change what I just broke down in plant quantity and size. It would just mean I do a little less defoliation, and I would have more room to get in to work.