beg to differ with ganja man. more light is always a tradeoff with too much heat. the 400 gives you 6,000 lumens per square ft of light, which is plenty.
yes, but heat would be a significant issue in such a small space, and the light would have to be farther away from the plants to avoid burning them, you'd need alot of ventilation and the gains from 400 to 600 might not be worth it
depends on the temps of your intake air. maybe. but unless you want the flexibility to grow in a larger space the 400 gives you the recommended amount of light for weed.