That's a lot of light! You have 4 square feet. 400 watts will be bright, and hot in there. That hood cannot be air-cooled, so I would get one that can be, should you want to add that later.
I also wouldn't worry about the MH conversion lamp. Using the enhanced spectrum HPS lamp all the way would be better because it has more lumens.
This is listed towards the end "future upgrades including air-cooled reflectors" so potroast was right it is not air cooled that is an upgrade. And you will want that in your enclosed area because heat build up will be a problem.
That light is good for that area, i got the HPS only version. While it's not set up to be air cooled.. i have a fan blowing right on the hood and it cools it well..