That error you're getting with preflash validation error is from trying to flash bootloader firmware older than your bootloader. You cannot downgrade your bootloader, and as such your device will report a 'security version downgrade' message. From the evidence, it looks like your device used to be on Nougat (B1:07 bootloader version, so December 2017 security patch or newer), and you downgraded to Marshmallow as identified by your device's build fingerprint. Thus, I'm guessing you took the December 2016 Nougat OTA update and hard bricked?
If you tried the flash file, you likely have a basic bootloader, and now require a full firmware flash of newer firmware than you currently are trying to use. You could flash only the GPT and bootloader from a newer firmware than you have (I recommend flashing the most recent Nougat firmware to get around that error message, NPJS25.93-14-18, April 2018 firmware), however, if you proceed to use stock OTA updates without full flashing you will likely hard brick again.
I would strongly recommend you download at least the most recent Nougat firmware from here
https://xdaforums.com/moto-g4-plus/how-to/stock-rom-npjs25-93-14-4-march-1-t3608138 and use that to flash. If you do not wish to erase your existing data, you can follow the following flashing commands instead of those in the mentioned thread, however, I would prepare to factory reset your device if you do encounter instability.
Code:
fastboot flash partition gpt.bin
fastboot flash bootloader bootloader.img
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