One of the sites I work on had 262 404 errors in early December, I recreated the sitemap submitted and the 404s then were 130. Now I have again recreated and submitted the sitemap and the 404 count is 130. I am unable to understand why submitting a completely new sitemap and removing old ones is still causing 404s. Why are 404s showing?
Are the URLs actually 404ing? If you copy the URL into your browser, are you given a 404 page, or the correct 200 page?
If they are 404ing, you will need to 301 redirect the pages in order to 'fix' the 404s, not just change your sitemap. Once the pages are indexed, they may stay indexed for any length of time, regardless of what your sitemap says. Even if you remove them from the sitemap, the will still technically be 404s as the content is still not found.
If the URLs are NOT 404ing, then the most likely explanation is that Google (or whatever search engine) has not recrawled the sitemap/URLs. You can resubmit your sitemap in Search Console, and also submit your homepage to the index to ask Google to re-crawl it (https://www.google.com/webmasters/tools/submit-url) and its links.