using a 400 i would have 9 plants in there, grow them to 1 ft flower them and they will get to around 2 feet when finished and give you a couple of ounces each. thats a very short veg time with a good return on 400watts. you will have plenty of head room to dissipate the heat so you will get less of a temperature worry
if you were to have 2-3 plants you will have to veg them for longer to fill the space and without pruning they will be too tall by the time they have filled the width, pruning adding to the veg time also