Depending on how well you take care of it you can get 10 to 16 ounces off that 400w.
Low Stress Train it, FIM the main four colas and then manipulate each of them so that a new one is higher than the last highest one every day (supercrop). Get rid of all the popcorn junk at the bottom before it turns into a sucker bud so that all the goodies flow to the top most buds. Grow it slow and cold. Lots of sea kelp. Cut out some posterboard and put it on the top of the pot so some light is reflected back up to the underside of the canopy.
There are so many things you can do to maximize yield. Just keep the temperature down and air blowing hard in that amount of space or youll be fighting bugs and powdery mildew for sure.
Also: I have a Lumatek 600w adjustable digital ballast. I ordered a 1000w and got this but thats neither here nor there. What is of import though, is that it is dimmable to 400w and 300w. You can install this ballast outside your grow area to minimize heat and dim it to 300w to maximize potential while lowering energy waste.