[Noishe]

The thing about 404, is that you can both return 404 as well as an error document that has the structure the author is looking for, which means there is no reason why you can't return a 404 status.

Got 404, returned body is json, returned body is an error object, good to go.

Oh and, by returning a 404 you help intermediate caches know that they should not cache the response.