No canonical URLs for paged discussions

Idan

I can't see the canonical tag for paged discussions (ie. /p2), FURTHERMORE I can access pages that ends with /p3, /p4 and they are accessible even that they don't have content yet. How can I solve both:

1) Enable canonical for paged discussions
2) Return 404 for pages that don't have don't have content yet.

Thanks

Idan

I really need to enable the option to return 404 for pages without content, like P5 when their the discussions didn't reach that page number