To boot a Motorola G4 Plus (Athene) into bootloader or fastboot mode, shut down the phone by pressing the Power button, then shutdown.
Once the phone is shutdown, wait a minute, then press the Power + Volume Down button together. I prefer to press the Volume Down button while the phone is off and then press Power button and keep both pressed for a few seconds. Then release the buttons and you will be in fastboot mode.
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