[Moto G4 Plus Questions & Answers] echo92:
1)You do not require root to install a custom ROM, just an unlocked bootloader and the appropriate TWRP (generally, 32 bit for Nougat custom ROMs, 64 bit for Oreo custom ROMs as of writing - check the thread for the custom ROM to verify).

2)You need a PC to flash TWRP onto your device. Afterwards, you can flash custom ROMs without a PC - you just need TWRP. However, having a PC is useful for troubleshooting and for reverting back to the stock Motorola ROM.

3)If you're on Nougat, ensure you're on the latest NPJS25.93-14-13 or NPJS25.93-14-15 firmware before flashing a custom ROM, else you may run into fingerprint/radio issues. With the backup, once you've flashed/booted TWRP, back up all partitions except cache listed in TWRP (these should include system, data, fsg, oem, persist, but may depend on the TWRP version you've flashed). Also, back up your internal storage to your SD card or cloud storage, or at least the key files.

4)As I understand it, unless you're having issues with stability or you want a really clean start or it's recommended in the custom ROM flashing instructions (which supersede any of this), you don't have to erase your internal storage. I think a factory reset should leave your internal storage alone, but you will want to back up regardless.

Besides, if you do not have an SD card to hold your custom ROM zip/TWRP backups, if you wipe your internal storage you'll also wipe your TWRP backups and your custom ROM installation file, leaving you with no system nor files to restore your system unless you have a computer. However, there are cases when wiping the internal storage has resolved issues, so you may wish to do so, but ensure you've got everything you need moved off your device. I would suggest buying a decent SD card (UHS-1 or UHS-3 grade, 16-32 GB or larger), if you do not have one already, for your zips and backups.

5) To the best of my knowledge:

Official custom ROMs - custom ROMs that are mainly provided by a central team of developers, and adapted for devices by maintainers (developers for a particular device). Generally, these controlled releases from the central team - must comply with their requirements (e.g. Lineage OS Charter) and maintainers are not allowed to add major features without consulting the central team. You'll generally expect similar, uniform releases between devices of a particular custom ROM, perhaps with minor tweaks here and there.

Unofficial custom ROMs - developed by individual maintainers or a small collaborative group, may be separate from the central development team. Able to add features onto custom ROMs without needing to consult, however, this may mean features on one custom ROM are not present on another due to individual tastes. Can be quicker to fix issues depending on the developer (as they're not bound to centralised releases).

Regardless of official or unofficial custom ROM status, there's such a wealth of high quality custom ROMs out there for our device that the best thing to do is to see which ROMs attract your attention and try them out. Provided they've been updated recently, the custom ROMs should work - just pay attention to the first few posts in the appropriate thread for any outstanding bugs, and use the search function to see if there are any reports of bugs with the latest builds. Above all, have fun :)

EDIT - rahulroy.e1004 is too quick for me...