If you issue a response.setStatus(404) from a servlet, you expect as per spec that the response is left untouched. No error page written. There is sendError for that.

Unfortunately, ErrorReportValve reacts on line 158
> if ((statusCode < 400) || (response.getContentCount() > 0))
>            return;

If you take a look at the StandardHostValve, it invokes error handling on line 316
> if (!response.isError())
>            return;
On errors only (correct behavior).

The ErrorReportValve must react on response.isError() == true only. Regardless of the status code or the content count.
The Javadoc does not limit this to status codes >= 400 but is says that reponse.sendError() has to clears buffers. As far I understand that, regardless of the response body is disposed and the error reponse is always written.