I read in a post that 4th rotary axis must be assigned A-axis
I’m using duel ball screws and my A is slaved to my X.
 do I need to change the A to B or can I assign the 4th axis B
Thanks Rusty

If wanting to view the rotational axis in the toolpath window then I think you will need to swap them about. If not then I dont think it will matter.
Having said that I have never messed with a 4th axis so maybe some of the others who have will chime in with better info.
Hood

Rusty,

I agree with Hood, swap your axis so that your rotary becomes Axis A (4). This way you can easily zero the rotary and check it's position or view the program limits etc.

Tweakie.

I did what Tweakie said because when posting for the rotary axis, post will output A axis as rotational move and don't want to mess with

Mastercam post processor to modify it.

Jeff