post processor for which software?
if it is already g code it should run as long as the arc codes are correct. and it fits on your table. the preview on the interface won't show A axis movements. if the program is 4 axis simultaneous motion, it will rotate around one of the axes, usually the x or y. if the rotary axis is only being used to index the work piece it isn't as critical. alignment with the fourth axis makes depth absolutely critical, as errors are doubled. if you are working between centers, make certain that your centers are perfectly aligned or it can cause problems too.
I use my fourth axis for both methods and use Mastercam to generate my geometry and g code, a standard fanuc post will work pretty well with some minor adjustment