1)Download the Lineage SU addon (from here
https://download.lineageos.org/extras - you want the arm 14.1 zip) OR magisk (from here:
https://forum.xda-developers.com/apps/magisk/official-magisk-v7-universal-systemless-t3473445 ) Only download one, please and if you want to switch, you ideally want to restore your backup or dirty flash the custom ROM to restore the Lineage kernel.
2)Reboot your device to TWRP.
3)In TWRP backup your device. Keep this backup safe.
4)In TWRP, go to 'Install' and flash the package you downloaded.
5)Wipe cache and Dalvik in TWRP after the flash.
6)Reboot.
7)Verify you have root.
You may also wish to browse the Lineage 14.1 thread for our device here:
https://forum.xda-developers.com/moto-g4-plus/development/rom-cyanogenmod-14-1-t3522101 Please use the search function by the page numbers to quickly search the thread for information