No need to relock the bootloader, but you must revert back to unmodified stock firmware on your device before updating. Without seeing what error it is, it's difficult to say. A locked bootloader strictly enforces having the unmodified stock furnace, which is why an OTA update is more likely to work.
Try this:
1) download the NPJS25.93-14-18 firmware from here:
https://forum.xda-developers.com/moto-g4-plus/how-to/stock-rom-npjs25-93-14-4-march-1-t3608138
Only use this if you have not updated to Oreo and you are still on NPJS25.93-14-18.
2)unzip the fastboot ROM, delete any old stock ROMs from your ADB folder and copy the stock ROM over.
3) assuming you had NPJS25.93-14-18 already on your device, flash the firmware with the following instructions - no need for the GPT/bootloader. Ensure you have your data backed up, as you may have to factory reset if you encounter instability.
Code:
fastboot flash logo logo.bin
fastboot flash boot boot.img
fastboot flash recovery recovery.img
fastboot flash dsp adspso.bin
fastboot flash oem oem.img
fastboot flash system system.img_sparsechunk.0
fastboot flash system system.img_sparsechunk.1
fastboot flash system system.img_sparsechunk.2
fastboot flash system system.img_sparsechunk.3
fastboot flash system system.img_sparsechunk.4
fastboot flash system system.img_sparsechunk.5
fastboot flash system system.img_sparsechunk.6
fastboot flash system system.img_sparsechunk.7
fastboot flash modem NON-HLOS.bin
fastboot erase modemst1
fastboot erase modemst2
fastboot flash fsg fsg.mbn
fastboot erase cache
fastboot reboot
That should get your device into an OTA friendly state without erasing your data. If that doesn't work, I suggest waiting for the Oreo fastboot ROM.
