What kind of 4th axis? Which axis does it rotate around?
Gearhead Mike made a turntable that allows him to turn round parts using a spinning mill to do the cutting. This is a “C” axis (rotates around Z) No Cutter Compensation G-codes G42 & G43 for Shapeoko?
I’ve seen a few guys made indexing rotaries that allow rotating the part to cut multiple sides.
Some manual, some controlled by swapping out the Y or X axis from the controller.
IF the part is mounted left to right so it rotates around the X axis, this would be an “A” axis rotary.
I mounted a little Dremel Lathe to my table & zeroed on the centerline of the lathe and used just my X & Z axis to turn a part. This would also be a “A” axis rotary.