You ideally would go with a 1000watt HPS and you would need ventilation. If ventilation is a problem than 400watt is good. But if you are only going to use one buld you ideally want HPS.

Your space sound GREAT.. it sound like it would be better to grow more than one plant.... maybe 8-20 plants. If you stick with your 400watt bulb you should keep your plants short(400watts will not penetrate very deep beyond the canopy). Flower them very early... ideally you should let you plants get about 12-18 inches tall. clone them... let them grow 7-14 days and then flower them...

good luck : )