you should be able to yield that much from a 400 watt light and just remember if growing from seed you will be looking at about 3 months maybe a little more from seed to harvest.
if you have a seperate space for mother plants and to start clones vegging you can reduce that time to around every 2 months for a harvest.
if you did decide to go with the 1000 and use the whole space you should have no trouble yielding over 1 lb with most strains and with some experience and high yielding strains some are pushing close to the 2 lbs per 1000 watt light with out using co2 just to give you a rough idea on yield if you went with the larger light.
bejay Reviewed by bejay on . HELP! 250 HPS or 400 HPS??? I will be flowering 4 plants, maybe 8, in a 4'x8'x8.5' high space that is well ventilated...Want to know whether I should go with 2- 400 watt HPS or 4 250 Watt HPS? Rating: 5