AC Infinity is a game changer.
In the wold of fans, back-pressure is right after CFM in importance. The mixed flow fans (Infinity, Vortex, etc) handle back-pressure better than axial (cpu fans).
Fun fact: one foot of the ‘slinky’ style flexible ducting we all use is equal (in air resistance) to 10 feet of sheet metal duct of the same diameter.
For your setup a 4" AC Infinity is more than enough… Unless you are thinking of expansion down the road.
Here’s some data I generated when playing with mine.
Where the Infinity ‘really shines’ is the controller… both temp & RH regulated
Your RH has a tendency to spike after lights-out due to falling temps. This can lead to mould issues in later flower.
My setup right now is max temp = 82F, min temp = off, Max RH = 53%, min RH = off
max alarm temp = 85F, max alarm RH = 58%. fan on setting = 4
(82F sounds high but my leaf temps are 74 ~ 76F)
So what happens?
During the day the fan regulates the temp to 82F by changing the fan speed from 0 to 4 (slow change in speed as it ramps up and down) at night, the max RH is controlling the fan.
Why is there a max setting and max alarm setting?
That’s the safety net!
If the temps creep up past 82F (fan running at 4) when it hits 85F the fan shifts to level 10
Belt and suspenders
No surprise my recommendation is “get a AC Infinity”. Even though you don’t need it, I’d suggest a 6" as this will allow you greater flexibility later if you decide to reconfigure your grow areas. The 6" pushes twice the air flow (400CFM vs 200CFM) for 30$ more.
Cheers
G