I am not aware of a "4th axis part diameter DRO" nor even a "4th axis part diameter" in PP. Mach3 has such a kludge; depending on your application inverse time mode (G93) may be useful.
Or did you mean the diameter that can be entered on the Settings/4th axis screen for 4th axis related probing? I didn't realize that that did anything other than make probing faster.