The G4 came out in 2017 (other than adding standard features to the ES model), & it has remained pretty much the same. Outside of adding cruise as standard in 2019, I don't see why a 2017-19 G4 would get any different mileage than the current 2020 Mirage. As far as I am concerned, there isn't an older model of the G4. When the updated 2017 Mirage appeared on the market, the current G4 appeared. There may have been some minor tweaks to the G4, but nothing that warrants a significant change in my opinion. Outside of adding more standard features (making the ES more like the other trim levels), these cars haven't changed since 2017.
If the updated 2021 Mirage & G4 comes to the States later this year, the front ends of the two cars will more alike. Subcompact Culture (small car blog) shows the two cars side-by-side.
http://www.subcompactculture.com/201...mirage-g4.html
None of this answers your question, but I don't think it can be answered until the new models arrive. The new updated models will have similar fronts with different rear ends.