Moto X4 Android variant has received Android Oreo 8.1 couple of weeks back, now the unlocked variant and Amazon Prime Moto X4 variant has started to receive the Android Oreo 8.1 update. The update carries the version number OPW28.2 for both variants.
As part of Android Oreo 8.1 update the devices gets the features such as dark and light themes similar to Pixel devices and includes May 2018 Android security patch. Update can be manually checked by heading to settings-> system-> software update. Update OTA rollout will take couple of days to hit all the devices as the update will be rolled out in phases.