I suggest get a 6'' fan and buy a 2x4 tent that has 6'' vent holes.
4'' is minimum and enough in theory, but you're gonna have to run it at full power, it's gonna be loud and possibly shorten the lifespan if working 24/7. And a 4'' might not be enough at removing heat and humidity depending on your environment. Especially with a carbon filter and ducting with bendings which lower the volume of air.
However, with a 6'' fan you can run it on less power, it will be quieter and work for longer, and you always have that extra capacity if it gets too hot and humid. And you can get a bigger filter to completely remove the smell. Just make sure to match the air capacity of the fan and filter. The filter can have a bit larger capacity, but not less than the fan.
I run a 6'' fan in a 2x2 tent, and i have to set it at almost half speed to fully control the environment. And it's still a few degrees warmer in tent, and about 5-10% more humid than the lung room. And that's with Migro Aray LED, which runs extremely cool comparing to other lights.
Mars Hydro gets pretty hot like a toaster on max wattage, at least in my case it was getting hot and smelled like burning electronics.
6'' fan is not much more expensive than a 4'', it's worth it.