shadow notification on right side of the screen
The Moto G4 has an issue with screen retention. Basically when something is on the screen in the same place for a long time, it will "burn in" and leave an after-image for a few minutes.

It seems that Motorola shipped the G4 with a screen that doesn't deal with screen retention very well, and it especially happens when the screen is warm.

In the end I had to flash a custom kernel (ElementalX) to the phone and use CF.Lumen to reduce the "Daytime color filter" settings to R=0.769,G=0.769,B=0.769 with the KCAL driver. This reduced the chance of screen retention significantly for me in exchange for screen brightness, but it still happens when the screen gets really hot.