A 400 watt lamp will light a 5'x5' area. So, I imagine that a 600 watt hid would work nicely in a 4'x4' area. The 1000 watt lamp would probably increase the yield, but by how much? I have a 3'x4' area, and a long time ago I asked the man at the hydro store if I could use a 1000 watt lamp, and he told me that it burn up my plants. Heck, I even have trouble at times (summer) keeping it cool using my 400 watt hid.
Not too long ago I was at the hydro store and I saw this really cool water-cooled reflector. It had a 1000 watt lamp in it, and it didn't seem to be too much hotter than my 400 watt lamp. That might do the trick for you. I wouldn't use a 1000 watt lamp unless I knew that I'd be able to keep the temperature down. I think that without excellent ventilation and exhaust your closet will be too hot with a 1000 watt light.