I think these and the 2016 retooling should all be G4 it's just not a big enough design change to warrant a half generation; size and proportions are the same etc. However I see these new demons as closer to G4.5 than the 2016 designs.
The difference from G3 to G3.5 is huge, and the reasoning between calling it G3.5 not G4 was because it was the same characters right? Well that's not going to ever change now, Hasbro finally has an optimist prime and a bumblebee of MLP so I doubt we'll ever see anything besides the M6 in different art styles again.
It might be hard to make stick but a numbering system for each retooling sounds the best for organizing as mentioned above