Hi Joe,
I am no expert in CNC, but I can tell you a little about what I have been able to do which has worked for me. I initially built a 3 axis router table, using Mach3 as the control software. I later expanded the table's capabilities to include a 4th axis. After looking around the internet for software which would work on a 4th axis, I tried CNC Wrapper. Basically, it takes a 3 axis G-code file, and substitutes either the X axis, OR the Y axis movements into 4th axis movements (depending on your specifications when you initially setup the software). The CNC Wrapper program also has stand-alone capabilities, which will allow you to do basics similar to what you have described. (I.E. turn square stock into round stock). However, just as it would be done manually in preparing a piece of stock for turning on a wood lathe, it is best to cut miters along the length of the stock, thereby changing it's beginning shape from a rectangular, or square shape to an octagon using a table saw. I have since experimented with 3D G-code files which were created using MeshCAM Art, then converted using CNC Wrapper, to generate G-code which was then loaded into Mach3 and "Wrapped around" round stock pieces on the 4th axis of the machine.