4 axes systems are generally used as follows: 3 axes for x, y and z, and the 4th axis would turn the work piece being milled, or to turn the router/spindle for reach in specific applications.
(4) NEMA 24 425 oz-in stepping motors
(4) Drivers (3.0Amp 24-40 Volts, 1-1/64 microstepping)
(1) 36v 8.8a Power Supply
(1) Interface Board (USB or Parallel)
Choose From 3 Interface Boards
Parallel Board: Works with Mach3, Mach4, or Linux-CNC. Parallel Port and Parallel Cable Required on PC running a 32 bit OS. Software can be purchased from BuildYourCNC.
Planet-CNC USB Board DISCONTINUED : Works with Planet-CNC Control Software ONLY.
Mach3 USB Board: Works with Mach3 ONLY. Connects through standard USB port. Mach3 can be purchased from BuildYourCNC.