I think you'll need to get a new CNC control box, which controls all 4 axes from a single port. I don't think they'll sell you individual boards. The "USB upgrade" (a UC-100 motion controller) is also going to be required if you don't want to keep using a parallel port. You can get Mach3 or 4 or some other CNC control software if you don't want to keep using MPS2000.