Man, that space is perfect a little SCROG, The light will distribute beautifully over all the nugs nearly equally if you do it right.

The Light:
The absolute optimal light for that space would be a 600W HPS and a 400W MH in a dual light hood with cooling.
Now assuming that Money is an issue. I would personally continue with the light you have. If you want 2 lights for the different photoperiods (I would), then i would buy the 600 W MH and use it for flower and use the 400W for Veg.

If you don't know what SCROG is, just do a search and you will see all kinds of howto's.

I would do 2 plants under whatever light config you choose. It won't take much time to fill a 4x4 screen with 2 plants, that is for sure.

What kind of yield do you want your system to produce? I always like to design around how much i am looking to end up with.