For historical reasons, the Urchin 3 product used content from Netscape and Apache error logs to provide specific references for 404 status hits (Page Not Found) in the Pages->Status/Errors report. There are constraints on processing error log data in Urchin 3, namely that it assumes that all the generated errors have a timestamp of the previous day.
Urchin 4 does not use data of any sort from webserver error logs, as all the information needed is gleaned directly from the webserver access logs. Urchin 4 provides much more comprehensive and accurate reporting in the Pages->Status/Errors report based soley on data from the access logs.
Consequently, it is strongly recommended that webserver error logs not be processed with Urchin 4, as there is no beneficial information for Urchin to process in this type of log. To the contrary, processing error logs with Urchin 4 may actually introduce spurious, unwanted hits with odd timestamps into the reporting data.