That 10" x 3" (30 sq/in) box ducting has a sectional area match of one 6" duct (28.27 sq/in), then your really hurting yourself reducing it to one 4" thru the wall, your down to down 12.56 sq/in). Add the ducting CFM resistance with a few bends your really hurting your system. If I were you I would rent a 8" diamond concrete hole saw, make a plywood guide to center the concrete hole saw to start the first 3/8" then punch out both of those existing 4" holes to 8" diameter. One for the cold air inlet ducted to the very back of the room and near the floor the other to your exhaust hood. That 746 cfm in line duct fan sure looks like winner to me if it were my brewing room as those high cfm numbers are without any ducting added. Even 10' of straight 8" ducting added to that 746 cfm fan will add backpressure and reduce the final cfm numbers of that fans ratings before you start adding any bends plus screens at the outlets preventing critters from entering from the outside. You already stated your having sweating walls from a short boil time wait until you have a 90 minute boil. I would be more worried about the heat buildup on your wine bottles stored in your brewing room also. The 8" duct with 50.26 sq/in of cross sectional area, you take this with the additional flow resistance of the ducting plus a few bends, bends kill your CFM's faster than about using any step down in duct sizing. Your system is clean and top notch in design plus equipment I would hate to see it ending up with a bad venting flaw. As mentioned by another members reply to have a flaired out hood vs straight sided plus all the other good features like the condensation drain gutter with line off the hood. Build it once right is far cheaper than twice I recall was also posted above. A Milwaukee Hole Hog would be a great powerhead to turn that hole saw, use tape and duct seal to direct the slurry when adding water to cool and flush away the cement as this would be a rather quick coring process. Two people, a hand square for a straight hole plus plan on getting a little dirty and wet. With the first class brewing system you have why go cheap at the very end here as your comfort will be a big factor with future brewing with a good vent system.
I'm thinking it would be great idea to have a inline duct fan on the fresh air side also for a constant and even air pressure in the brewing room as outside the door to it. I can spend your money rather easy, it's fun also. JMO's as I was taking classes to become a "Tin Knocker" many years ago and we had to figure out the correct CFM's, fan sizes to match the duct length runs.