If you decide to go with the X-Crave take a look at the V-Carve Pro software option (Discounted if you purchase it with the machine)
V-Carve has a video tutorial for 2 sided carving with their software. (Care one side, flip, carve the other side) I recommend taking a look so you can see what is involved.
While it is possible to modify the X-Crave to add a 4th Axis that is going to be a very advanced project.
For more info check out the various 4th axis threads.
There is a simpler rack and pinion setup that could work for rotary carving with minimal modification. I may give this a try myself.
I think the V-care Pro software will support this via the “Wrapped Rotary Axis Toolpaths” feature.