Just from observation, not scientifically tested.... but....
as oldgrayhair seems to be seeing, smaller pots can restrict plants size, so, as a previous poster suggested, you might be better planting 4x per [bigger] pot.
Personally, as it is something as important as Marijuana, I would spend whatever it takes to maximise the potential available.
Hydroponics use less rootspace than soil, why not live life dangerously and do your first COCO grow.... ?
From doing SoGs a while ago... what really counts, IMO, more than "seeing roots growing" is seeing the plant is growing, vigorously. This applies more with Hydro where the initial dramatic acceleration of growth can, if ignored, lead to wildly differing plant heights and subsequent loss of yield. [Average, total, do not get seduced by the stars of the show, it is about achieving a uniform crop, leading to the maximum possible total.