This is old but thought I'd chime in as I have recent experience with 4th axis and will post for your info.
The 4,5, and 6th axis, or A, B, and C on the G100 are exactly the same as x, y ,z. If you don't want it to move, don't give it a command. If you have the axis setup as any other axis it will be locked into whatever position it's in or move in step with the other axis as required.
In my case I'm using the A axis for the extruder on a 3d printer mill conversion. When 3D printing its a pretty slow process limited by how hard you want to accelerate your axis. It's kind of interesting how the G100 firmware will proportion just enough extruded plastic at whatever the max speed and accelerations I can get from the x and y which are limited by the large amount of mass they move. If you watch the extruder gear you can see it speed up and slow down to match the net x, y velocities. The A axis position is always increasing as the plastic extrudes out. When the head stops the A xis backs up a fraction of a mill to keep the nozzle from dripping. If you want to know how much plastic you extruded just look a the A axis number and multiply by the filament diameter.