400 watts cover a 3 foot x 3 foot space. your space is slightly larger.
i am using a 400 watt in a 4 ft by 4 ft space for 10 plants and its working fine. Its crowed as hell but they are all budding, i had 10 outta 14 turn out to be female, i was shooting for 6 females but i'm not complaining about 10!
i'd rather have a 600 in there. - you'll be fine with a 400, but will have a better yield with a 600 watt.
i got mine from HTG Supply- cheap, fast and discrete packaging