I've wired mine up for the 4th but dont have a 4th to try it. Most of the servo chassis had the spare slot for the 4th, but the controller doesn't have it available. At least from what I've seen.
If you pull your servo transducer board it should say 5 axis if it has the capacity to run a 4th axis. One of the axes is dedicated to the spindle which is why it needs 5 to run 4. I'm sure this varies somewhat depening on the age of the controller but thats what I remember.
There are two ways to go with the wiring, one as indexer only, and the other as a full contouring 4th. With or without brake.