Thanks Fastback and Woodnthings. My DC is a Grizzly G1029 2HP unit rated at 1182 CFM. There is a 5" inlet which has a y-fitting to two 4" ports. I like the idea in the video where the motor is mounted to a wall. I may do that in my DC closet, which for my setup will allow me to minimize some connections in the closet. I think I figured out a solution to my original problem. I have one almost empty joist bay, but I do have an HVAC branch and a drain pipe at one end (as previously discussed). I tried to get the 4: S&D pipe through there. It's close but just too tight. But a few minutes ago, I tried a 4" DC hose (which has a smaller diameter of course) and it just fits. So I am thinking of using just enough hose, maybe 2-3 feet to get through the congestion and connect each end to the S&D pipe. I would keep the ground continuous by jumping the short section of hose.
Does that sound like a reasonable plan? I should be able to stay at 4" throughout the shop until very close to each machine.