cfm question again..

i got a 400w hps lamp and i made a cool tube would a 4" duct fan that has 80 cfm be enough to cool it through 3 feet of ducting?
the temps outside of the bulb will be hot around 90 degrees if not more, the light is seperate from my grow box which is already ventilated now i just need to cool the bulb

or would i have to take a 250 cfm one?

You should rig it up and find out. It might work, it might not. Good luck.