Another vote for the AC infinity series of fans. I run a 6" on a 5x5 tent with LEDS. I bought the S series which I believe is their cheapest option, and hooked it up to the 67 controller. The 67 controller is really the next step in automation from them. In my current configuration I can set upper and lower fan speed limits as well as triggers for temp and humidity. It's not a perfect set up but it is very nice for all the more it costs. I have my upper fan limit set to 50% and it more than adequately exchanges air in my space through a carbon filter.
I would however advise spending the extra money on the 69 WIFI controller. I am waiting for them to be in stock to purchase one as they are brand new. The 69 does everything the 67 does and more. I believe it can control up to 4 AC infinity devices on separate programs AND hooks to wifi. The downside of the 67 controller is it only has bluetooth access. The cost is double (49.99 base for 67, 99.99 for 69 WIFI) but I believe it is well worth the cost for the added features.
All that said, I have an identical 4" fan from spiderfarmer. They look the exact same but smaller. It produces noticeably more noise even at a low setting, and the noise is very high pitched. It's extremely annoying and if noise is of concern I would watch closely which type of fan you purchase as all are not created equal, although visually they may look similar.
