Thats what it sounds like he has.
You dont need a motor with this connector. All though you might have to figure out a few things to get it hooked up. Those microphone connectors are what I used to wire my motors and you can get them at any Radio Shack or electronics store.
The stepper you just need to size to fit the job at hand. But you need to know what your controller is able to drive. The type driver card should be easy to find out and then you can buy a motor which fits within its ability, and one that will have enough power to run its intended job ( run a fouth axis in your case).
This is not hard. What controller did you get with your mill? Chances are others already know which driver it is using. Then what rotary table are you going to use? Again there are probably others that have used it and this will help guide which stepper to buy.
Jess
GOD Bless, and prayers for all.