By sideways, I mean horizontally.

Almost all of the light being produced by an HID light is emitted at 90 degrees from the tube. A lot of industrial fixtures burn the lamp vertically, sending the light out paralell to the floor. This works well to disperse the light over gigantic areas, but not so well for indoor horticulture purposes. Anybody who's old enough to remember big 7 & 8 ' parabolic reflectors will remember that they burnt the bulb in the vertical position. Modern ( horizontal) reflectors are much more efficient, unless you're lighting up, say, a Walmart. ( If you are going to light up a Walmart, remember to douse it in petrol first )

A 600 would be perfect for a 6x4- unless you wanted to stick 2 600's in there. I would hang the reflector so that it was paralell with one of the four foot walls- this should give you the best distribution pattern. It may be more conveniant, depending on door placement, whether or not you need to run ductwork, etc, to hang so that your bulb is paralell to the longer walls, but you will have a tradeoff in the canopy penetration at the ends of the room. If you go for two lamps, you're essentially lighting 2 3x4 areas- making the 4 foot wall the long wall, so I hang the bulbs paralell to the six foot ( or the two x three foot) wall.


Also, I should mention that if you're going to adapt old vertical fixtures to new horizontal ones, please please please make sure that the socket that you use is rated for at least 5kv and that the cable you use is rated for at least 600v- the current coming off the ballast is 600 volts, and 1000w @ 600v will turn an extension cord into a heating element- like in a toaster.

Glad to be of help- assuming I'm actually being helpful, and not just a grumpy old fart