Hey welcome to the forum. You’re asking the right questions in the right place.
This post is worth a read:
I’ve always admired that setup. The basics are that air flows better in smoother passages that are larger in size. So yeah larger diameters are better for airflow,as is a smooth pathway, free of internal rough surfaces and curves.
As for larger fans? Generally they push more air, but also they can run at lower RPMs that are inherently quieter. If you go to 6 or 8” size ducting and fans the limiting factor for your airflow will likely become the 4” opening from your glowforge, as well as any internal fans/cooling systems (in the pro) that cause resistance.
HVAC pros will have a lot more knowledge, there are ways to calculate these things. Heck there may be an online calculator that will tell you your expected airflow these days, I haven’t looked in a while. Conventional wisdom says that anything up around 200 cfm for a performance model is adequate. More is generally better,