ventilation setup in small tent

A good 4" fan is probably enough, but may need to run over 50% which increases noise. Might be worth the 6" cost, especially if you have any intention of scaling up in future. (Get an "EC" motor. They will come with 0-100% or 10-100% control.) the key is to match cfm of duct fan to your cfm-rated fitler. I don't know what maximizes effectiveness, but i'd wager less than the maximum... one more reason to buy a size that you can run at 50% or less relative to tent size. They are fairly standard, if i recal.. the 4" filters usually are rated for ~170 CFM, which should be roughly what any decent 4" duct fan is. A little negative pressure is all you need. If using a carbon filter, use a small length of duct for the carbon filter to be in the tent, but can have the fan outside of the tent to save room. Again, much easier to do when the fan is a bit on the strong side, rather than 'just enough.' I don't have any carbon filters, but none of my fans are inside my tent.. don't want that shit in my way :P

You do not want to just buy the biggest fan you can fit in there, that will use up valuable growing space. There are diminishing returns. but generally you should have some headroom with your fan. For a small grow tent like that you do not need anything larger than a 4 inch fan. You don't want to over power your fan it will cause too much negative pressure. Unless you need it to be super quiet, it will be worth the extra money. but for most people 4 inch is fine. i have mostly 6inch fan i use up to a 120x240cm tent. I really should be running 8 inch for my larger tents. they do run at high speed often.. but i have a 4 inch for my small tent and works perfectly. 4sure

In my 60×60 tent, I’m running a cheap fan that can move 185/250 m³ of air (switchable). It came as part of a €100 set with a filter and works perfectly fine. Nowadays, I’d spend a few euros more to get a fan that can be controlled via an app and a filter that allows you to refill the activated carbon. To work effectively against odors, it’s important to replace the filter every few grows or, if possible, replace the activated carbon. Also, the connection between the fan and the filter is often prone to leaking odors, but the rubber couplings available from hardware stores do a good job of preventing that. If you want to use ducting, I’d still position the fan so that it protrudes through the tent opening and only attach the duct outside the tent. This prevents the duct from drawing in “false air” from inside the tent.

Buy biggest you can fit in, then when you get bigger tent, your setup will be ready without update. Plus running 5-6 inch fan on 30-50% of power is way more silent than 4 inch on max. Don't forget to measure with carbon filter ;)