lucy24 - 1:01 pm on Jan 5, 2012 (gmt 0)
My very concern though is that, given this kind of case, will Google still consider the canonical tag placed in the header of these pages?
A 404 is not a noindex or a nofollow or a robots.txt. A 404 is they really can't find it. You need to figure out why the page doesn't seem to exist for them. A 403 would be understandable; a 404 suggests that it's time to fine-tooth-comb your htaccess. Look for something like a UA-based redirect to a page that doesn't exist any more, or a non-lethal misspelling.
That's assuming you want g### to get to the pages, right?