I’ve seen movies (I think like Pet Sematary 2019) where you can hear PANNING sounds from one Atmos ceiling speaker to the next utilizing all 4 separate Atmos speakers. IOW, each of the 4 Atmos speakers is independent and discrete = 4 discrete channels.
If you just duplicate the front 2 Atmos to the back to make 4 speakers, it’s still just 2 discrete channels of processing. The Panning will be just left and right, not front-back, left-right, front-left to back-right, etc.
The potential is there for very cool Atmos effects utilizing all 4 independent discrete Atmos channels.