Hey man, so let's look at this. For your 4x4 space the 600 is perfect, I think any more and you would either be running it on lower setting or having to add CO2. If you are going to upgrade to a 5x5 I would get the 650, if sticking with the 4x4 go for the 600. It gives you 37.5 watts per square foot so without CO2 is about as optimal as you will get I think. As for heat, my heat sync is right now at 33.5, my canopy temp is an even 25. The light is set at 50% and about 28" from plant canopy. I am flipping to flower in next week at which point I will raise the power up every day for a week until it is at 100% and at about 24". That is what Steven at HLG stated and the plants seem to love it. If you do go with either light let me know and I will forward you the instructions I got when I spoke to HLG. Here is a pic of my 4 plants 2 minutes ago when I measured my temps for you. The 2 on the left are Shiva Skunk, the 2 on the right are Maui Waui. They were all topped once. Oh and another note, I think the 6" fan should be fine for either tent. I am using a 4 inch and it is very easy to create negative pressure at less than 1/2 power so I think the 8" is probably overkill in my opinion. I know the HLG's are more expensive but no matter what, price aside, I cannot see you being disappointed with either purchase. I know I am super happy with mine.
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