For 4 plants you won't need a 5x6 area, the light will probably only need to cover an area of 2x2 (1sq ft per plant) so a 250 watt hps will suffice or a 400 watter if you want to go a little over the top but IMO either a 600 or 1000 would be complete overkill for 4 plants. For an extra $50 i'd definately get the cool tube, your heat will be a lot easier to manage with it.

Good luck.