turn that oven up another 15 degrees and you have las vegas,i was right,its hot out baby! like riding in an oven...albeit an oven set to just over body temp,but an oven none the less...
two six inch fans will handle 4 hoods just fine....
if sucking air no more than 2 lamps per fan,i would stick to that even if blowing the air through since the last lamp in line is going to have really hot air blowing across it...
"T"s suck for airflow,you wants "Y"s to connect ductwork...
depends on your fresh air source temp as to what your room temp will be,you will have to set the house a/c to like 70 to maintain a 70-ish degree flower room with 4 lamps...its not the bulb heat,its the infrared the lamp generates coupling with stuff enough to warm it up,like the air namely...so it would be best to get the filtered air out of the room if you can,it will set up better airflow if the flower room is being evacuated as well as refreshed...