No, actually, I DON'T know what you mean. If you don't know what file has been requested, it's difficult to know how to redirect it to a permanent link (other than your own 404 page which is NOT a permanent redirect, is it?).
You COULD use mod_rewrite to redirect anything that does NOT match your directories, your files or your redirects to send to 404 but Apache has the more elegant solution: ErrorDocument 404 /404.php where the first 404 is the error code and /404.php is the ABSOLUTE link to your 404 handler.
Client Request RedirectedI don't think that you want anything but a 302 is reasonable until you can identify the 404s from your error log and create a redirection (if applicable).
300 Multiple Choices
301 Moved Permanently
302 Moved Temporarily
303 See Other
304 Not Modified
305 Use Proxy
Personally, I use the ErrorDocument statement (and redirect to my sitemap). I've had other sites link to my site to pages that simply do not exist (presumably, in an attempt to annoy anyone who would visit my site) but I don't have the time to chase these bogus links - ErrorDocument and a well maintained sitemap take care of that for me.