the footprint for a 400w h.i.d. is 4x4'. 2 would work great.
fluoros work well as stated above, but you already have 2 400w so go for it. Just make sure that you have good exhaust to keep a healthy airflow; And a good fan across top of canopy.

footprints by size.
250w 2x3'
400w-4x4'
600w-6x6'
1000w-8x8'
latewood Reviewed by latewood on . Recommend height for MH lighting i have some seedlings starting i was reading that having ur mh light to low will fry ur seedlings and then having it to high causes it to stretch , from what ive read 5 feet is the norm, but i was wondering what u guys thought about 4 feet high for the seedlings and im not to sure after they start growing like during veg and bloom how high i should keep it. thx guys, my average temp is between 72-78 night and day so any help or suggestions would be nice, i know i gotta test it for my self as Rating: 5