Thank you for this post, I too had a nightmare trying to re-register a 4th gen kindle. For anyone else who is having this predicament, you won't see the option to restart in any obvious place, you need to press the settings button, go into settings, then once in settings, press the settings button again and you will see the options to factory reset and restart.
When I tried registering, it kept telling me my account was not recognised in their records and to try a password reset. I tried my amazon account and my mum's and I had the same problem with both. That's because what the device will not tell you is that you need to go to your emails and find a one time password that you then need to put into the password field
. After a long chat with an Amazon agent and me just randomly going to my email to delete the reset password link she sent, I saw a load of emails with a otp from amazon.
This finally got me registered and recognised. But then cam the problem of syncing my books. I tried a few times and it kept showing that I had 0 archived items. I went to the website and sent a book from my kindle library to my kindle manually. Still my kindle was empty. I then did a restart from the settings menu and when I switched it back on, sync finally started working. Anyway, it was a mission and a half but I finally got it working. I'm so grateful for my straightforward Kobo Libra 2.