Without having to reference the criteria (because I think it is not necessary to answer the question), you are probably struggle with the question because of a couple of reasons.
Firstly, it is possible for there to be more than one way of classifying something, and if the criteria doesn't allow for this (or isn't flexible enough to adapt) then you should consider customizing it so as to be more fit for your purpose.
Guidance and error management can both fall under the bigger perspective of messaging and notification to the user, which allows the user to feel in control of their actions (in their interaction with the system). So I don't see this as a problem that you can't fit it neatly into just one category.
The other thing is that there are ways to implement a 404 page that leads to a more positive user experience, and ways that can make the user more confused and frustrated. So it is not so much redirecting the user to a 404 page that is the problem, because at some point your system will probably generate an error that you can't necessarily address immediately (e.g. the server is down).
But as with all good error handling and management best practices, if you can:
- identify the source of the error (i.e. is it caused by the user or
- provide some idea the nature of the error (e.g. is it an expected error caused by things like validation or something unexpected and outside of the system) and then
- give some ideas of how to resolve the issue or perhaps next steps
Then your users will thank you instead of clogging up your support system/mechanisms.