A 4" fan IS sufficient IF you have a air cooled hood for your light.
I run a 400wt hps with 2, 4' 4bulb T5 fixtures one on each side of the walls in my 3'x4'x6 room
I use a can filter that is hung from the ceiling.
when it is really hot in the summer I have to shut the bottom 2 bulbs off in the T5 fixtures.
I will upgrade to a 6" fan someday.
But a 4" will work. I have been running this for 3 years.