OK, so the pot shafter adapters arrived today after about 3.5 months. This was because of some sort of disagreement between the US Gov't and Deutsche Post (translated, Donald Trump did another stupid thing).
http://www.banzaimusic.com/Axis-Converter-6-3mm-4mm.htmlSo, the adapters worked exactly as I had hoped. I was worried that because the pot shafts were D-Shafts (not completely round), that the adapter wouldn't grab enough of the shaft to get enough grip, and they would slip. But the adapter needed to be pressed onto the 4mm shafts were a fair amount of pressure, so they were already a tight fit even before the set screw on the knobs added some additional pressure. These things are fantastic and I highly recommend them.
In the pic you can see a finished EQ on the right, and on the left, you can see two pots with the adapters pressed on, and two without.