The update is currently being rolled out in the US
It appears that Motorola has changed their mind after they said in their post that the Moto G4 and G4 Plus will not receive Android Oreo. Recently, there is confirmation that the Moto G4 Plus is receiving Android Oreo in the US, though there is no word yet for the smaller Moto G4.
The update will most likely be the last major update for the Moto G4 Plus, as the phone initially shipped with Android Oreo when it was launched in 2016.
On the other side of the spectrum, the Moto Z3 Play is reportedly receiving Android Pie as promised. The said update delivers all of Android Pie’s key features such as gesture-based navigation and adaptive battery.