I have actually heard from several sources (local hydro shop dude included) that a 6" fan will be quieter than a 4" fan when running at the same CFM. So if a 4" would cut it running at full power, a 6" speed controlled down to the same CFM would be quieter while accomishing the same job.

The fan will be inside the cab. The cab will be in an unused bedroom, and I'm not breaking any laws out here in CA, so noise is not ALL that important anyway. My plan is to build a diy duct muffler if the noise bugs me.

I've seen a few posts showing various DIY duct mufflers, but here's my idea.... It's quite similar to how DIY carbon scrubbers are made, but instead of a cap on the end I'll have an opening. Picture about a 15" long section of 10" diameter steel duct. Inside that 10" duct, there will be a 6" diameter "roll" of hardware cloth (held in place just like with the diy carbon scrubber. The space between the mesh and the 10" duct will be filled with foam, batting, or something of that nature. A 10"x6" reducer will be one end, and a 10" flange will be on the other, and will be sealed up to the exhaust hole. I think it'll greatly reduce that whooshing sound air likes to make.