6 plants seems about right for a 400W HID, I suggest you keep the plants fairly short as a 400W HID isn't going to penetrate a canopy very well, the top foot or so is going to get the most penetration but I have seen some pretty nice 3ft plants grown under a 400W HID, long veg/flower time with lots of popcorn buds in the end though, I have seen guy's grow just over a pound with a 400W CMH & a waterfarm DWC system, VERY IMPRESSIVE! With a 400W I would recommend you do a SOG, use 2-3gal pots (Superoots Air-Pots are the best) and do about a week veg time until the plants get about 10" or so and then start flower, you want to keep the plants short (between 20"-30" when finished) and remove any side branching, you end up with one beautiful fat football sized cola per plant that can weigh up to several OZ's (dry weight, no joke) if grown hydroponically that is. The beauty of this is that although you harvest less per harvest you have practically no veg time and usually a slightly shorter flower time so you harvest more times per year so you end up with more weight in the long run, the whole point of growing cannabis (and not to mention the funnest part) is harvesting the fruit of your labor, so why not do it more often and get more buds IMHO! Whatever you decide good luck and keep us posted.