Nope, they really mean Black Butler #4. You can see it going up and down the list if you track back through the weeks. It seems to take a while from the time a book debuts to the time a book shows up on the list.
For example, Finder 3 has been available for over a month now, right? Maybe two? But it's only just debuting on the bestseller list this week. I think it takes time for the books to reach all the sellers that would then be eligible to be counted for the NYT list. It's my guess that Akadot sales are not reported to the NYT.... ?