[Moto G4 Plus Questions & Answers] echo92:
Which adb/fastboot utility did you install? If you install Minimal ADB 1.4.2, on my computer it's present under C:\Program Files (x86)\Minimal ADB and Fastboot. For 1.4.2: https://www.androidfilehost.com/?fid=745425885120698566

If you're trying to flash TWRP (ensure you've got either TWRP 3.0.2 or a more recent version, 3.1.1 for example, and that it's for athene devices), you could follow this guide: https://xdaforums.com/showpost.php?p=67031805&postcount=3 Also, double check you've unlocked your bootloader before attempting any of this: https://xdaforums.com/showpost.php?p=67031801&postcount=2 Bear in mind unlocking your bootloader will wipe your device and void your warranty.

Once you've flashed TWRP, then reboot straight to recovery - this is to ensure TWRP is not overwritten by the stock recovery. Once you've booted to the recovery, then TWRP should stick and you may reboot as normal.

EDIT - instead of flashing TWRP, you may wish to boot to TWRP (and look at step 4 below):

Copy the TWRP image file to your adb folder.

Boot into the bootloader and connect your device to your computer.

Open your adb client and verify your device is communicating by executing 'fastboot devices' without the quotes. You should get a response with your device's serial number.

To boot in TWRP, execute in the adb terminal 'fastboot boot <name of TWRP image>' without the quotes. E.g if your TWRP is twrp-3.1.1-athene_shreps.img, then the command you'd use is 'fastboot boot twrp-3.1.1-athene_shreps.img' (without quotes).

Afterwards, you'll be able to flash LineageOS and GApps. The main instructions for flashing a custom ROM are usually listed in the first post of each custom ROM (here's the link for the official LineageOS: https://xdaforums.com/moto-g4-plus/development/rom-cyanogenmod-14-1-t3522101)

If you have your SD card and internal storage merged, then unless you can save the encryption key from your stock, your SD card will be wiped/inaccessible. SD cards mounted as portable storage appear to operate okay under both custom and stock ROMs. Thus, back up your SD card to your computer. You may wish to re-format your SD card as portable storage (and from what I've read on the Lenovo Motorola forums, that might help your SD card last longer, as it's not being subjected to encryption as it would be with adoptable storage).

1)You'll be flashing the LineageOS and GApps (can use either OpenGApps or BeanGApps) zips using TWRP. You need 7.1 arm Gapps (32 bit).

2)Download LineageOS and GApps to your device. Do not unzip them.

3)Reboot to TWRP recovery. Tap the Backup option on the main menu.

4)Back up your device in TWRP (make a note this is your stock backup). If you want this backup to be OTA friendly for stock updates, then do not 'swipe to allow modifications'. Back up the stock ROM, verify you've got the backup then reboot to TWRP, and then 'swipe to allow modifications'.

5)Go back to the main menu, and tap the 'Wipe'.

6)Under 'Advanced Wipe', select Dalvik/ART cache, system, data and cache. Double check these are the only ones you have ticked.

6a)Swipe to wipe.

7)Go back to the TWRP main menu.

8)Tap Install and locate your LineageOS and GApps zips.

9)Flash LineageOS first and then GApps.

10)Reboot to LineageOS.

If your device screen goes to sleep whilst in TWRP, press on the volume keys to wake up your device.

If you wish to revert back to near stock, you can flash your TWRP backup of your stock ROM (though this will revert any messages to the point when your backup was taken, you may wish to use Titanium Backup or MyPhoneExplorer to backup). If you decide to revert completely to stock, then if you were on the June 2017 stock update, please only flash the June 2017 or later stock firmwares: https://xdaforums.com/moto-g4-plus/how-to/stock-rom-npjs25-93-14-4-march-1-t3608138