Reception hall that takes 400 guests & allows outside catering in BROOKLYN
You could try the Irondale Performance Center http://www.irondale.org/rental-info.html . They say that they have a capacity of 500, but I doubt if they could fit 400 seated.
SkylightOneHanson is the former bank space in the Williamsburgh Savings Bank Building. Quite high end. Says it will take 500 seated. http://www.skylightnyc.com/