what is more optimal for a 2x4x8 a 400w or 600w without added co2 dimmable ballast. i know i would be able to get the 400w closer without bleaching but would i get a bigger yelid with a 600w a few inches further away ?
I run a 600W in a closet almost exactly the same size as yours (4.5lx2dx7h). I have an air cooled hood and a 440 cfm exhaust fan cooling the light. There is also a XL carbon filter on this fan. I purposely bought a dimmable digital ballast so that if I couldn't control the temp I could turn the ballast down to 75%(450W) or 50%(300W). Running at 100% (600W) the temp in the closet (doors completely closed) is 4-6 degrees (F) above ambient temperature. With the air cooled hood, the heat is way easier to manage. I shopped around and bought my whole setup (600W dimmable digi ballast that runs both MH and HPS, XL air cooled hood, MH bulb and HPS bulb all with a 1 year warranty) for $200 on ebay. HTH
The 400 will serve you well and heat will be more manageable. But the 600 will yield better as long as you can dial in your ventilation setup like Ganja Girl did. If you can vent into an attic, problem solved. The 600 will give you approx. 50% more bud if you can handle the constraints. For the narrow/long format, I'd use a light mover but those are a PITA to vent. Can be done with a curtain traverse rod mounted beside the mover to hold the ducting.