I'm new to CNC machining, so please forgive the ignorance of these questions:-) Can you get good detail and undercuts for sculpture in the round (i.e. statues) using a 4-axis milling machine as opposed to a 5-axis? What’s the difference between them in what they can do?

4-axis are way more common in any case compared to 5-axis. 4-axis is XYZ with a turning axis, 5-axis is much more complex and the moving/turning directions depends on the setup.
use the search function and you will find some examples.

You're better off either cleaning up the undercuts by hand, or milling your sculpture in parts to be able to mill undercut areas and then assembling.

5 Axis machines and the software needed, are far, far more expensive, complicated and difficult to tram/align then 4 axis machines.

If it has to be milled, has to be milled in one piece, and can't have any unmilled undercuts, try to farm it out. Cheaper and more timely in the long run.

You can also look at Rapid Prototyping.

timbot1982,

The size of your parts can be much larger with a 4 axis setup than for 5 axis for the same money. Big 5 axis costs BIG money. I have done a lot of 4 axis rotary engraving and I think it would be the most practical way to go. :)