Is the chuck reversible on the 4th Axis unit?

Yes you can remove the three tabs from the chuck and put them in the opposite way to allow for a larger opening for larger stock, up to 2 inches in diameter.

To remove them, simply turn the inner ring behind the chuck counterclockwise and the threads will push the chuck tabs out.  Then you can flip them around and while putting pressure on them turn the inner ring clockwise and they will be threaded back in.