This was a serious issue prior to Google's revelations about such errors. According to Google's official blog post, If you deliberately deleted or removed those pages from your site or forum, you should not worry about it. The search bots will automatically drop those URLs from the search index in few days and it will not affect your site's search rankings.
In Google Search Console, just go to the "Crawl Errors" and select all those 404 pages and "Mark as Fixed".
If those 404s were not deliberately deleted or removed by you from the site, then you should redirect it to the right path. It's very simple.