Good rule of thumb. 4X 5 = 20 sq ft. Good lighting would be 50 watts conventional lighting per sq ft. so we are talking 1000 watts of HPS, I would suggest 3 400's for these reasons : 1 if you stagger the turn on time by 1 minute each, the current inrush will not have a chance of tripping a 15 amp ckt. 2. smaller light, closer to a plant. 3 you can achieve better light dispersion with 3 sources than one big ass hot spot.
4.Burn out a bulb and you are only 1/3 down till replacement. 4. Ballast will last longer and caps cheaper to replace. 5. In a pinch, bulbs can be had from most home depot type shops or electrical supply shops for security systems. Good luck, don't even tell yourself