Error hooks

[squishi]

Hi! I want to execute something when a visitor of my forum hits a 404 error page.

I found three plugin hooks that are related to errors:
error_fetch
error_generic
error_nopermission

In which php files are they?
Which one would I use?
And how would I test in such a plugin if the page is a 404 error page?

I want it to execute only on 404 errors. So I don't want to add an error check on regular forum pages.