i dont know how a 4" would cool a 1K HPS. I get the plants within 12 inches and dont have any residual heat from the light. Another guy Doco on here uses a 6" as well, along with a speed controller to lower it. The filter will reduce the amount of air the fan moves, so remember that. I wouldn't risk all my plants over a few bucks. I have a speed control and it lets me dial in the exact amount of air i want to move. I can tune it to use the light as a heater during these cold nights here in cali.