i would get a 3x3 thts there footprint pretty much..a 4x4 is like the maxxx for a 400w..
i ran a 400w a few grows ago in a 3x3 by 7ft tall tent with only a duct booster fan ($25 on amazon) venting from the top and bottom vents open always, and a fan inside mixing it up..temps never got over 85F...