[Moto G4 Plus Questions & Answers] echo92:
As a tip, if you wanted to flash a custom ROM, you do not require root whatsoever. You'd need an unlocked bootloader and the correct TWRP to flash the custom ROM.

It's odd that the backup you restored was the Feb 2018 security patch, is it possible you had another backup in addition to the April 2018 update? Since the backup was made before you rooted, it is expected you do not have root.

As for your questions:

1) Try this (I have not formally tried it, so be careful) https://xdaforums.com/moto-g4/help/troubleshooting-twrp-boot-loop-ota-t3714325 This should break the TWRP bootloop. If you wish to revert back to the stock recovery, you can find the recovery (pulled from the respective stock ROM): https://drive.google.com/drive/folders/0B-GUyOfb8OgzcUl1NUtHYWNIR2M If you've flashed the Feb 2018 firmware from your backup, I'd suggest flashing the Feb 2018 stock recovery, even if you were previously on the April 2018 firmware.

2)Alternatively, you do not have to root your device if you wish to re-flash the stock ROM for a clean start (or any ROM on our devices). If the above link doesn't work to break the bootloop, flashing the stock ROM should help.

The April 2018 NPJS25.93-14-18 stock ROM is here: https://xdaforums.com/moto-g4-plus/how-to/stock-rom-npjs25-93-14-4-march-1-t3608138 You may choose to use the following fastboot commands, instead of the commands in the stock ROM thread, if you wish to keep your bootloader unlocked and try to keep your data, but be aware that you may have to factory reset in case of instability. Thus, before using the following commands, I would recommend booting to TWRP and back up your TWRP backups and any important files on your internal storage. Then reboot to bootloader and begin flashing the stock ROM.

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

This should formally flash your device back up to NPJS25.93-14-18, it should also remove TWRP but you can re-flash that easily.

If you're still intent on rooting the stock ROM, please bear in mind that on stock Motorola Nougat, you must flash a custom kernel first before rooting. What that guide failed to mention is that those rooting steps work for stock Marshmallow, but under stock Nougat, the anti-rooting measures are much stricter. As such, attempting to root on the stock Motorola Nougat kernel will result in boot failures. If you flash a custom kernel such as ElementalX, that will make rooting straightforward (I'm rooted on NPJS25.93-14-18 with ElementalX 2.0.1 and magisk 16). Of course, this will make you ineligible for OTA updates unless you flash back the same stock ROM. However, with rooting or flashing custom ROMs, you may want a copy of the latest stock ROM to hand anyway. For rooting on stock Nougat, here's a guide: https://xdaforums.com/moto-g4-plus/how-to/guide-root-moto-g4-plus-supersu-android-t3587918 You can substitute magisk for SuperSU depending on which root manager you prefer.

Again, you do not require root at all to flash a ROM (custom or otherwise) on our Moto G4/Plus devices to the best of my knowledge.