If you're sure that specific URLs should not exist on your site, 404 errors won't hurt your search performance, and you can safely disregard them. It is crucial to ensure that these and other invalid URLs return a proper 404 HTTP response code and are not blocked by the site's robots.txt file.

Here are some guidelines for dealing with 404 errors:

1. Determine if it's worthwhile to fix the error. Most 404 errors are not worth fixing since they don't affect your site's indexing or ranking.
2. If it's a submitted URL that resulted in an error, it's worth addressing.
3. If it's a deleted page with no replacement or equivalent, returning a 404 is appropriate, and the error should stop showing up in reports after around a month.
4. If it's a wrong URL that never existed on your site, you don't need to worry about it unless it's a commonly misspelt link. 404 errors should be removed from the report after around a month.
5. If the URL is a commonly misspelt or alternately spelt word or URL, map the variations to the same page on your site using redirects or another method.
6. If the URL was submitted for indexing and displays an error, inspect the URL to determine where it was submitted from and update the sitemap if necessary. If the content has moved, add a redirect.
7. If you have permanently deleted content without intending to replace it with newer, related content, allow the ancient URL to return a 404 & 410.
8. Don't create fake content, redirect to your homepage, or use robots.txt to block 404s since these actions make it more difficult for search engines to process your site's structure properly. These are known as soft 404 errors.
9. If you frequently encounter a 404 error for a particular URL, create a redirect for it.

Remember that submitting a URL removal request using the URL removal tool will not eliminate the error from the report. Additionally, issuing a 300-level redirect will delay the recrawl attempt, possibly for an extended period. If Googlebot encounters an unexpected URL, such as a URL generated by JavaScript or Flash files, it may produce a 404 error in the Crawl Errors report. Still, this error should not affect your site's crawling or ranking. Google is working to prevent this type of crawl error.