Personally I think a Pro 1000 for a 4x4 is a lot of overkill if you run it at 1000W. A 600W would be a bit on the low side (<900 umol avg).
For 4x4 actually a 750W/825W would be a great choice. Fortunately the Pro 1000 is still very efficient at 750W (1.86 umol per watt, 1.92 umol per watt at 825) . Don't forget that you get about 25% more light than with an air cooled reflector. If you use a 1000W at 750, then do take the time to run the lamp at least 100 hours at 1000W for it to stabilize.
At 750W you almost get almost 1000 umol avg @ 55 cm from the light. If you want more: you have it, but then take some more distance. Do you need to tune down: you can. @ 825W the output is about the same as a
Hortilux 1000W HPS(a good lamp!) at 100% in an ideal aircooled reflector (95% reflector efficiency, 6% filter losses).
@lex: more and more houses and apartments have separate 240V group available for high power appliances. In some cases you already have two phases in your house, but they are split into 2x120V. Take a look
here. You can probably ask your utility company if you have it, or if they can make a 240V group. It has many advantages, such as more efficient appliances, less copper needed for your lines as the current is half of the 120V current for the same power and also important: less heat dissipation in the ballast (at least 30% less than 120V ballasts).