I used a 4" fan in conjunction with the internal exhaust fan. My duct was 25’ to the fan, and another short run outdoors. When the internal fan failed, it was almost immediately obvious the external 4" was not sufficient (debris accumulation was rapidly increasing.) I removed the internal fan and grate, still was not sufficient.
I switched to the 6" fan, which is still at the end of the 25’ of 4" duct, but now a 6" pipe from there out the house. It works better (less debris accumulation) than the original setup did.