Http 404 errors are counted in the CAS against every component of the page. If you want to know if it was against the base HTML of a page then you have 2 options.
1 - Assuming you have >12.3 you can configure availability setting to treat 404s against the base component as a failure. All 404s will be counted but only thoses agains the base component will be treated as a failure. You the correlate 404 errors with failures.
2 - Use an ADS. The ADS not only tracks error codes but if you enable hit level details it will tell you the return code for each component of the page.
Hope this helps.