Acoustic ducting hummmm I'll have to look into that, sounds interesting. I believe inline fans come in 2" increments so you won't be able to get a 5", and if you do you'll probably spend a lot more cause it's not a standard size. As for a 4" vs 6" that's a tough one, and is up to you. The 4" will more than likely do the job well, and if it's a quality item it should last awhile. Its probably the route I would go, having gone through these issues of trying to keep the walls off my plants. If you do go with the 6", just be aware that it will suck the sides in, so you'll need to account for that.
If smell is not an issue, you can skip the carbon filter and save a few bucks that way. For me the smell is no issue, and I found my appartment didn't smell bad at all, and the wafts I would get when the door was open was quite nice lol. I got 1 just incase, and I'm not using it on this grow. I doubt I will replace it.