If you want to go with LEDs check out Guru Grow Lights. They have a six month return policy so you don't have to worry as much about them not yielding since you can always return them.
Its probably best to go with a bit more then four in the case that one dies you won't lose as much. You could try nine then choose the best as space gets tight.
Glad to see someone else going with LEDs. I made the switch and I am kind of wondering when they will catch on with the general growing population. There are alot of pretty crap products in the LED market but I went with four of these bad boys and their delivering quite nicely so far. I still am...
I don't know about HPS but if you live in Vancouver you could drop by Guru Grow Lights to check out some LEDs. Saves you some money and the having to deal with the heat issue.
Can I ask why you decided to go with HPS? You might want to give LEDs a try cause you would save around 2000W. There is a good guide here on the advantages of LEDs. I went four 300W LEDs and they are suiting me fine so far.