Client error (4xx)
What it means
These URLs do not exist or refuse access. Any link, sitemap entry or bookmark pointing at them is wasted, and the link equity they would have passed on is lost.
How to fix it
For each URL decide which it is: content that moved, in which case 301 it to the new location; content that is gone for good, in which case leave the 404 and remove the links to it; or a mistake, in which case fix the URL. Do not redirect everything to the home page. A soft 404 is worse than an honest one. Check the Inlinks panel to find the pages that need editing.
Finding it in CrawlStat
After a crawl, open Issues and pick Client error (4xx). Click Show all in Pages to filter the Pages table to the affected pages, or Export affected to hand the list to whoever owns the fix. Audit ID: status_4xx.