My assumption is that you could mix 301 and 404 lines in .htaccess files so that you can redirect specific pages and then use a 404 redirect to mop up the rest to a custom 404 error page. But seemingly not?
My assumption was something like that below, but no it doesn't seem to like it.
This line doesn't mean "send user to custom error document". It means "respond to requests for this named document with a 404" presumably leading to exactly the same type of 500-class error that arises when you neglect to poke a hole for your custom 403 document.
:: detour to check something ::
Thought so. The numerical status code in mod_alias behaves exactly the same as the argument to the [R] flag in mod_rewrite: if it's outside the 3xx range, the server simply returns the specified response.
Which reminds me that when I saw the subject line I was afraid it would be a question about mixing mod_alias and mod_rewrite in a single htaccess file.