For purposes of Urchin, "subreport filtering" means parsing out data for all sites on your server from one logfile. This ability is not explicitly available in the Urchin 4 product -- it was eliminated due to reliability and accuracy problems inherent to the technique. In order to duplicate function with Urchin 4, it is necessary to specify individual profiles for each site within your shared logfile, and use a profile-level filter to isolate data for each specific site. There is significant additional overhead in processing logs this way, and so for this and the other reasons mentioned above, it is highly recommended to have separate log files for each site on your server.