[Moto G4 Plus Guides, News, & Discussion] echo92:
The OTA update system is designed and intended for step-wise, forward-direction updates only To the best of my understanding, yes. I do not think that Motorola, and perhaps other OEMs, ever designed their OTA system to account for device downgrades, hence why in the OTA update description they mention you cannot downgrade after flashing this. Also perhaps why, at least for Motorola, you assume responsibility for anything that happens to your device after the first bootloader unlock.

At times, it will block regression in the interest of precluding the installation of an outdated and insecure bootloader (as you described and warned).

I don't think OTA updates block regression of your device firmware, the regression block seems to be from your device bootloader. Your device bootloader will simply block flashing of bootloaders older than your current bootloader, which is half the danger of downgrading as this results in the mismatched system/bootloader that sets you up for a brick.

I'm not sure what you mean by this 'It is also dangerous to go around the OTA step-wise behavior and "flash ahead" to a ROM revision that embodies more than a single OTA step, because an older installed bootloader may not be compatible with newer ROM content.' though. If I've understood you, flashing a newer stock ROM in full (including GPT/bootloader) should be okay, provided the stock ROM is for your device variant/region. There's nothing to stop you directly flashing NPJ25.93-14.7 or NPJ25.93-14.7-3 (Dec 2017) onto your device, provided it's a full flash since it's a forward flash as you've mentioned - i.e. you're flashing newer firmware onto your device. This full flash does not have to erase your data, but always wise to back up. The fastboot files replace the required partitions with full copies (incorporating the OTA patches) rather than the block based patching of the OTA, so it's useful to perform and a mandatory flash for updating on downgraded devices. Thus, you'd be updating your bootloader to the same patch level by doing a full flash (of course, provided you trust the firmware enough). As you're full flashing (and in most other cases when you're flashing stock boot/recovery/system/OEM/logo), you'll be wiping TWRP, root and custom kernels/logo/modifications from your device, so no need to unroot from your root manager. The flash of stock files for those particular partitions should wipe out root.

Flashing ahead is not a problem, it's if you flash behind (i.e. flashing a stock firmware many OTA levels/builds older than your existing firmware and definitely if it's much older than your bootloader patch level) that sets you up for a hard brick. E.g. in your case, it'd be similar to flashing MPJ24.139-64/65 (Marshmallow 6.0.1) or NPJ25.93-14 (December 2016 Nougat), then attempting to take OTA updates. If you reverted to NPJS25.93-14-4 however, the directly preceding build to NPJ25.93-14.5 (as I remember), taking the OTA to NPJ25.93-14.5 may be safe as the OTA would be flashing the same bootloader files as what you have now.

As for builds: NPJ is Nougat (the first N is Nougat, as is M for Marshmallow and O for Oreo), whereas NPJS should denote a security update. However, Motorola's naming convention is somewhat inconsistent. AMZ_NPJ stock ROMs, or any firmware with AMZ in their title/file name are for Amazon Prime devices (with the locked bootloader, adverts on the lock screen, and cheaper than retail devices). You probably don't want AMZ ROMs on your retUS device.