AC Infinity CLOUDLINE T4, Quiet Inline Duct Booster Fan 4” with Thermostat Speed Control, for Heating Cooling Ventilation and Exhaust, Grow Tents https://www.amazon.com/dp/B06ZXWN3BG/ref=cm_sw_r_other_apa_BAiKzbDAD66WW
I’ve fallen in love with this fan. They make them several sizes, but they release them periodically. They are currently only selling this size, and of course it’s more affordable to buy on Amazon. If you need something stealthy, this is perhaps your go to fan. Visit their website to see when the other fans will be released.