I asked that a new forum be started for wood lathe and mills. I know that is not your thing but a lot of the things you do with metal will relate to wood. Anything you can post to the wood lathe section would be appreciated. With wood I will start with a square or octagonal stock. Assuming I want a stair end post that has some of the piece square and some round,. Do I run it as a mill or as a lathe? I think that there would less tear out of the wood in the area that transitions from square to round if it was milled. However the round areas would be faster if they were done as a lathe and the cutting was done with the side of the cutter. At the bottom of the page at http://turningaround.org/4_axis_mill.htm you can see part of the desired end post. How would you do it? Please post to the wood lathe forum.
Your machine is a 4-axis MILL. I suppose you could operate it as a lathe by replacing the router head with a rigid lathe tool, but the rotary axis is not fast enough to effectively cut as a lathe spindle.
1) If you start with square stock that is properly sized and you can position it accurately centered in the machine, then you would only need to cut the round features. Using your 4-axs machine, the rotary axis should be set parallel to X and then the part machined in 3-axis mode using rotary, X and Z axis only. Y movement is not used. You would use a milling cutter and cut the part while rotating the part 360 degrees and incrementally stepping along the X axis. This will result in circumerencial ridges resulting form the ball-end mill and the amount of stepover. Another way would be to mill all along the x axis and then incrementally step the rotary axis between passes, resulting in longetudinal ridges based on the ball-end mill radius and step angle.
2) If you want to machine the square features also, then you will need a slightly different approach. You could machine with the end of a flat end mill using Y-X rastering passes, rotate 90 degrees and repeat for 4 sides. You could otherwise use a long end mill tool and side cut the square features. This will give you a better finish, but require a very long tool, which is expensive and you may not be able to find one to fit a router motor.