We currently don’t have any threaded holes in the side of the cross pieces. We do however have holes in the top for soil board mounting that could be used to mount either a 4th axis or a jig to hold boards vertically for edge work.
There is 7 3/8in between the top of the cross pieces in the table and the bottom of the gantry. I would suggest mounting the 4th axis so the center is right at or slightly below the top of the table cross pieces. If you mount it above the table you will limit yourself on how big of a piece you can turn.