[Moto G4 Plus Questions & Answers] echo92:
1) As long as you flash the same or newer stock firmware as the latest on your device, you should be okay.

As of writing, if you flash the June 2017, Sept 2017 or Dec 2017 stock firmwares (NPJS25.93-14-8, NPJS25.93-14-10 or NPJS25.93-14-13 respectively) then you shouldn't run into issues and be okay to take OTA updates provided you do a full flash. By a full flash, I mean flashing GPT, bootloader, system, OEM, DSP, modem and the erase steps. If you're flashing the NPJS25.93-14-8 firmware, flashing the same firmware should do nothing to your GPT, bootloader or modem, but is a good routine to ensure your device is clean.

As you're switching from RR to stock firmware, a data wipe/erase is recommended - you may be able to use Titanium Backup or another backup app to preserve your app data, then restore it after flashing the stock ROM (though ensure the backup is off your device). You will have to root your stock ROM to use Titanium Backup however.

The risks come when you flash stock firmware of an older patch level than the stock firmware currently on your device (custom ROMs do not matter in this regard) - strictly speaking, it's not the act of downgrading but attempting to take OTA updates whilst downgraded that causes hard bricks. In most cases, if not all, I've noted that a mismatch of the bootloader patch level and the system patch level are what cause issues for most users. You cannot downgrade bootloaders, so downgrading leads to the following situation:

Say you're on the June 2017 firmware:

June 2017 bootloader, June 2017 system (or RR).

You downgrade to Marshmallow or an earlier build of Nougat (say in this case Dec 2016 Nougat firmware):

June 2017 bootloader (bootloader will block downgrades with a security downgrade error), December 2016 firmware.

You try to take an OTA update. However, OTA updates are only designed for updating, and appear to assume (from looking at the script) your system patch level is the same as your bootloader. If you did not downgrade, this assumption would hold. However, as you've downgraded, you'll get this:

March/June 2017 bootloader *, March 2017 system.

Due to the OTA having authorisation to flash to your bootloader without prior checks, the older OTA corrupts your newer bootloader (as denoted by the *), leading to a hard brick. We do have a blankflash at the moment to help with this, though the easiest way to avoid this situation is to only flash firmware of the same or newer build that what is on your device. Motorola does not support downgrading and assumes the user takes responsibility for downgrading.

2)In theory yes. However, if TWRP was allowed to make modifications before you took the backup, the subsequent OTA might fail as your system partition would be detected as not read-only. Furthermore, you'd need a stock logo.bin, so if you've flashed a logo.bin to hide the bootloader unlocked warning, you'd have to pull that from the NPJS25.93-14-8 stock ROM. TWRP to my knowledge does not back that up.

3)As long as you take your time, and have not downgraded/hard bricked in the past, you should be okay. Ensure you back up as much data with Titanium Backup as previously mentioned, and ensure that you do a full flash, so your GPT, bootloader and system are at the same patch level. As mentioned previously, having your device on the same patch level should mean you have fewer issues with OTAs and stability.

4)I'm not sure what you mean by the June patch, do you mean the June OTA update, or do you mean the June stock ROM?