What is a good 4th axis that is compatible with the XCarve pro
Well, the XCP isn’t actually setup to run a 4th axis, so whichever you get will require some customization to wire it in, to Clamp one axis in place and you’d need to unplug that Axis (typically the Y is removed to accommodate a rotary, but you could remove the X, totally up to you) . . . Then you’d need to adjust the grbl settings for steps/mm and use the rotary’s steps/rotation instead.
Unfortunately there just isn’t a plug and play kit out there for the Xcarve or Xcarve Pro.