Im running a single 1000w Sunsystems AC/DE setup in my 5x5. Veg with MH at 1000w flower with HPS at 1100w
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6'' ducting is fine my fixture temps at 85 degrees its not even warm to the touch even with the fan running on LOW. And I push the hot air to my basement. The fan creates negative pressure in the tent pulling air in through the bottom vents. The room/tent receives a constant supply of fresh upstairs air. You'll have to imagine ducting where the arrows are. And yes that is a 6'' fan attached to a 8'' filter. Bigger filter=better flow and longer life.
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4 oscillating fans on low/medium speed to keep the fresh air moving under the canopy.
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And one oscillating fan above the canopy.
This year was autos, getting everything dialed in. Next year will be photos. I just do one grow during winter so that temps are easily controlled and all that heat coming from the light goes to good use heating the house.
Anyway the point I was getting at is that single ended bulbs are a thing of the past. You would be much happier running a 600 or 750w double ended for a 4x4. Double ended bulbs are 20-30% more efficient so suddenly your 600w bulb (boosted by 10% to run at 660w if you please) is throwing down about the same amount of light as a single ended 1000w. I am extremely happy with the sunsystem AC/DE hood and I will be running this setup for years to come.