Probably only "need" a 4" system for that size. The 6" can move more air at lower and quieter speeds if the tent is in a location that would be noisy for you.
Do you already have an inline duct fan picked out or just need a new filter? I can say that the ACInfinity inline duct fans and controllers are top notch. Their filters are inexpensive but I don't think they do the best job. They are fine through veg but into flowering they get overpowered. I have one 4" and one 6" system of theirs.I have no other filters to compare to but I do not know if the price jump to some of the other brands is worth it. I have had to build an additional scrubber outside the tent that I vent the exhaust into.
I should be building a new scrubber in the next couple weeks. I can put together a tutorial/guide when it's ready. It is unfortunately a bit spendy but if stealth is important I don't think the price is too bad.