The black box runs Grbl on the same chipset as the carbide boards. It’s a 3 axis firmware with four drivers, just like most Grbl boards. The 4th driver is just for dual steppers on X, Y, or Z.
When you see Grbl 4th axis setups, they are usually replacing the X or Y axis with a rotary axis. You’d have to lock your x-axis in place, and connect the driver to your rotary axis. Effectively mapping the X to the rotation.
I’d like to see Carbide 3d be the first to adopt a beyond Grbl controller (open source).