well how much of the area do you plan on using? 4x4 is recomended for 400wtts and i would agree with it. u cannot just leave your door open, very bad idea. That light is blinding bright and will surely be noticable, not to mention you would have to remeber to leave it open or risk killing your crop. I dunno you can surely put in some ventilation, be inventive. You could even post a pic of the grow area and maybe some of us can help out with ideas.