What's the most appropriate HTTP status code for an "item not found" error page

Getting overly clever with obscure-er HTTP error codes is a bad idea. Browsers sometimes react in unhelpful ways that obfuscate the situation. Stick with 404.

A 404 return code actually means 'resource not found', and applies to any entity for which a request was made but not satisfied. So it works equally-well for pages, subsections of pages, and any item that exists on the page which has a specific request to be rendered.

So 404 is the right code to use in this scenario. Note that it doesn't apply to 'server not found', which is a different situation in which a request was issued but not answered at all, as opposed to answered but without the resource requested.

404 is just fine. HTTP/1.1 Status Code Definitions from RFC2616

204:

No Content.” This code means that the server has successfully processed the request, but is not going to return any content

https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/204

That's depending if userid is a resource identifier or additional parameter. If it is then it's ok to return 404 if not you might return other code like

400 (bad request) ‐ indicates a bad request
or
412 (Precondition Failed) e.g. conflict by performing conditional update

More info in free InfoQ Explores: REST book.

If the page itself doesn't exist, I'll obviously use 404.

What you said there is a bit confusing but I will have to assume that you're developing an API backend.

The problem is that whoever is consuming the API endpoint may be confused in two ways:

They may think the 404 returned is because the endpoint(resource) wasn't reached, Or They might think that the item or user being requested wasn't found.

So the trick is, how are they going to know which is the right assumption?

Well, the answer is simple. Always try to attach a body to any errors that you returned from code. Errors that are returned by the server automatically do not have a body. So try to attach a body which you can document so that they can be cable to use the content of the body to differentiate between code returned errors and server errors.

But in a nutshell, 404 is the right status to return, but try to attach a body to it indicating why 404 was returned.

An example could be:

// For illustration I'm just gonna use C#
Return NotFound(new { errorMessage: "Item requested was not found" });

Here, NotFound returns a 404 statuscode and the parameter is an object like { errorMessage: "some reason for the error"}. This way, you can always check if your error returned a body, and the you know it's returned from your code. Otherwise, the resource(link) wasn't found.

/**
* {@code 422 Unprocessable Entity}.
* @see <a href="https://tools.ietf.org/html/rfc4918#section-11.2">WebDAV</a>
*/
UNPROCESSABLE_ENTITY(422, "Unprocessable Entity")

Since it's a user-facing page always use 404. It's the only code people know usually.

For the api request, use 400 with the error message "No such user exists" or something along those lines.