Good luck on your SOG.
I might suggest you add another 315 as your outer plants will not receive the same amount of light as the inside plants.
I am running 2 - 315's in my 4 x 4 and loving it. 18 to 21 inches above the canopy.
From experience, if you try filling the 4 x 4 footprint with one light, the height you would have to raise the light will not give you the desired product you are looking for.
This pic taken 4/4 is a single 315 that I raised to get the plant to stretch so I could attach it to the begin filling the screen. The height is about 26 inches plus, don't remember exactly, and yes I got good stretch but it caused the node spacing to be to long between the nodes. Since I do heavy defo that does not effect this grow.
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This pic was taken 9 days later when I added the second 315. The height of the lamps are 18 inches.
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Now, that being said, I still don't get true coverage of the 4 x 4. With the lights side by side at 18" the actual coverage is approx. 5' x 3.25', due to the overlap for the 5 feet and the side by side alignment for the 3.25 feet. Now the higher you raise the lamps the larger the foot print but the less intensity of the light. I would not go above 21" for normal growing and prefer 18".
To get around the coverage problem and also my back problems, I built a lazy susan to hold the 10 gallon smart pot. It has rollers on the bottom that ride in the channels of the catch tray so I can move it in and out and also rotate it 180 degrees.
It sure makes it easier to work on this plant.
Taken yesterday 5/7 day 7 of flower, stretching has begun, the lamp is raised as needed to keep the height at 18".
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GR