Ima all over this... Here you go I'm lazy so you have to read.
"Maximum yield for a 400 watt MH"?!!!!
- And the consensus is: notably less than a 600w hps. And I think probably even less than 400 watts of leds! A 400w HID just doesn't add mass to the buds as the genetics can; the light cannot drive 'swelling' calyx's, because it does not throw enough flux. I can't even imagine getting more than a quap from a multi cola plant fucking hugging that whole bulb for dear life, for coldness and death at the margins are a whole freaking 8 inches away from the bulb.
And cut out the fucking hydroton. Always a bad solution. Its just a good entry-level "turn key" system to offer... In that space, you are right to attempt sog, but you are able to now only work the footprint immediately below your 400w HID reflector, that's it, maybe a total two foot square of useful irradiation for flowering: Budswell loves the warmer spectrums, even though the plant grows towards the cool blues. And the more you hit it with (the near reds), the faster and fuller they will grow... calyx's bursting out of calyx's layering protective resin glands as the floral clusters reach out to fill only richly radiated space. It's not all pistils to these flowers. It's having relationally sized fruiting bodies, calyx's from which to build on and up and fill out.
-Sog under a 400 will yield high quality medicine, but it will not yield. Think about it as increasing the surface area for resin head development. The plant doesn't have enough encouragement to continue it's bud swell and put on mass. Small plants low yield.
Switch to perlite/vermiculite mix. Drip feed a low ec of something nice; run to waste canna for instance or make your own nutes. Control the environment with ac and dehumidifier, and cover your pots, or add CO2. This will all help.
But not much under a 400 watt will still yet happen... A qp of sensimilla, not stems, at most, is all you can yield flowering from maybe 4 at most sog colas per light, keeping the light pretty tight, and you will still rather wish the final product was worth its volume than its weight... Which I do agree is partially true...
Add a couple of led flowering bars, like two 150's, and you'll start to see a difference.
60w of lighting per foot is the minimum calculation, but also remember that light intensity / photometric flux degrades inversely at a rate of the distance squared from the initial point source of the 'lumens' or what have you such that an object twice as far away receives a quarter of the light intensity. No reflector helps overcome this law.
400w HID especially won't give that kind of coverage. A 600 MH or HPS will, though not even yet to your full meter area edgewise. A full meter takes a 1000 watt hps. Not really a 6 and a 4 and not from two 4's/ Similarly, the 'canopy depth' of a 1k bulb is the whole plant. A 600 will perform adequately and with a thick canopy, but not on a meter scale. Not for consistent self pruning lower sog type buds. This is why 'anything less than' a 1000w is what it is.
A four hundred watt will not make for sog plants which compactly fill their given space, its a light for one or two flowering plants, even starting as small seedlings, which are not sexually mature at 6' either. This is an autoryder situation, and even those take time.
Stepping up equipment and space sucks, so fuck your question. Its your difficulty. A 400w MH isn't really a great start. Not with a room. A cab is different as its design accepts the high potential of negative use value.
Grow outdoors. Or burn some light, and pay the vendors for the privilege.
Treeth!