Small ones or Big ones?

Im flowering under a single 400HPS. I was wondering, is it better to grow 4 bigger plants in 3.5 gallon pots, or would it be better to grow say, 9 smaller plants in 1 gallon pots?

As long as theres not too much light for the plants, it doenst really affect it either way, as long as the 400w is absorbed by plants, the same amount of energy will go into making buds, using different amounts of plants is usually growers preference, growing style and height availability
What size space are you growing in?

I believe the testing showed that with near perfect techniques it proved that SoG growing produced SLIGHTLY more yield, but depending on MANY factors, they are pretty much the same.

That being said. With the 400w light (With a proper reflector) you will probably get better yields with smaller plants due to the lower power. The light will have less penetration. Means the lower branches on larger plants will not get much light and not yield much.

And with many smaller plants, you can have less veg times, and more harvests throughout the year. Look up Perpertual harvests.

Im just growing in a small bedroom, the light is in the corner, so space is a little more fluid. Makes it easier to control temps due to the larger room size.

Smaller number of larger plants.

Easier to manage 3 pots vs 9.

I would even consider SCROGing one plant.

I'd do a SCROG, but they flower in one room, and sleep in another. I run all lights 24/7 for vegging flowering and clones, but I just run 2 shifts for flowering. I take the 1st set out of the dark, and put them into the light, then I move the second set out of the light, and put them to bed. I effectively doubled my flowering space this way.