I adapted a 2.5" shop vac hose to the 4" vac pipe with a simple reducer fitting and a 2.5" coupler. The shop vac hose was a loose fit in the PVC coupler, so I roughed up both surfaces with coarse paper and glued the 2 together with liquid nails. I now have a 2.5" shop vac push button quick disconnect connection. The other end was also close but not an exact fit. The flared end of a shop vac attachment fit the sander perfect but was just a smidgen small to fit the vac hose since it was, well backwards. So I heated the now modified shop vac attachment with a heat gun & slid it onto the hose. When it cooled the 2 pieces fused together. Now the shop vac hose slides onto the hand tools with a firm twist.
BTW: Your 4" flexible hose will fit inside the female end of a 4" sewer fitting. It's a tight fit but with a little finesse can be worked into the fitting. You can see I did that in the first photo. I put 3 self tapping screws (set screws) in the fitting for a little extra hold. Also, I would use smooth wall pipe wherever you can get by with it rather than the flex hose as the ribs causes drag on the airflow. I have flex hose connected to the table saw because I'm always moving the saw around in my limited space. I'm using 4" PVC sewer pipe everywhere else. Another neat trick is 4" sewer pipe fittings fit inside 4" schedule 40 fittings. I used a combination of the 2 to make quick disconnects for the dust separator.
Hope that helps.
Last edited by pro70z28; 06-24-2018 at 08:38 PM.