what? Of course you need room for roots. 4 inches might be enough for small plants flowered immediately and in small netpots. But basically, the mass of the green part of the plant will not be greater than the mass of the roots.
It depends on how big your plants are. I was growig 3' topped plants and they'd use up most of the space (hudge dense mat of roots) in a 5 gal bucket by havest time.
