Yes, it can be done. I’ve replaced my Nomad controller with an EdingCNC CNC720 but the approach applies just as well to any other CNC controller, be it Mach 4 or otherwise.
The main trouble is exposing the Nomad’s motors and controls to some other controller. If you’re happy to tear apart the Nomad, you can just cut off the existing connectors and crimp on some ferrules or something. I wanted to preserve the original connectors so that I could use the original board still if I wanted to, so I made a breakout board that exposes the Nomad’s internals via standard screw terminals.
However the Nomad’s stock controller enclosure is very limited on space, so I ended up making a dedicated board just for the CNC720.
I can upload the files for my PCBs somewhere if you’d like to make and solder one yourself. The PCBs were $5 + shipping from China and the connectors cost ~$20 IIRC (mostly screw terminals, which are quite expensive if you buy quality).
FWIW, I’d happily recommend EdingCNC as an alternative to Mach4. The website sucks but the hardware and software are quite good and the price is pretty great. It’s also small enough, with a carrier board, to fit into the Nomad’s stock controller enclosure.
But before you go and do this, what is it you’re missing from the Carbide Motion controller that you hope to gain from Mach4?