Rule of thumb seems to be 30w-40w(atts) per square foot.
9'x4'= 36 square feet
36x30= 1,080watts total for a 9'x4'.
How true this is, i don't know. I'm just relaying what a google search is telling me:
https://www.google.com/search?client=firefox-b-1-d&q=cannabis+watts+per+square+foot
Now, don't forget, just because a light puts out enough wattage for a grow room doesn't necessarily mean it will be able to cover that room evenly. You'll have to do some research to figure that part out.