I looked at this with my father in law's redwing hit and miss engine. For his shaft it was possible to offset a 4 jawed chuck to turn the ends leaving a billet in the middle. By then transferring it to a mill he could indicate in on each shaft end, vertically as well is X and Y and then center drill the billet to be mounter in between centers. Saw part of the billet away and the use a grooving tool to turn the center of the shaft, however it was a lot easier for me to make him one to demonstrate the process of making it in pieces and jigging it to hold it in position to solder it. I can't answer the 4th axis question with any authority as all my 4th axis work has been in index mode, however the drive/encoder of the 4th axis will determine how true of an arc you can produce on your shaft. Deflection on the scale of his engine would be a serious consideration as well.