Are you talking about a conventional 4th axis on a vertical machine?
I've only begun to dabble in this myself, using.....you guessed it...OneCNC XR2. Do I sound like a stuck record or what?
But, basically what I am finding out, is that there is not a heck of a lot to doing it. I wish the advanced software was cheaper for the average guy to see what I am talking about.
There are 3 modes of operation for 4th axis:
Positional: this simply automates the indexing, so that you can then perform machining operations at that particular rotary position, with the indexer locked.
Live: using a 3d solid model and Solid Machining Technology, the OneCNC software can generate the commands to rotate the part automatically during a sole rotary axis command, or a compound linear/rotary command. The software creates the toolpath from the model's surface information.
Wrap: this is sometimes easier than using "Live". This would be because it is sometimes easier to lay out a flat map of the surface features of the part than it is to actually model it in 3d. In this mode, the software transforms any Y axis movement into the equivalent rotary axis command. This is suitable for drilling hole patterns (radially centered on the 4th axis), or for machining a flattened (or linear) surface model (like a large helical groove) that is easy to sketch out on the flat, and much more difficult to make a model of.
They claim it is the easist 4th axis to use, and while my experience with other software in 4th axis is nil, I can vouch for the fact that it is easy to pick up. Mind you, I've only been machining for 30 years, if you think that might have any bearing.
Still, I'm a noob to using 4th axis software, so that says something.