Hi Pablo,
What report are you using to find the 404 pages? Are they soft 404 or hard 404s? A lot of reports may return a 404 response for something like a failed file request which does not really impact usability.
If you have a consistent page name for your 404 pages, look under Analytics site content and filter by page title then search for your 404 page title - this will give you the actual failed page requests, you can then narrow down if they are internal page links or external.
It may be worthwhile checking what links are pointing to your 301 redirects and if there are links you want to keep, dropping them could have a negative impact on your site.
Regards,
Jason