The 404 Not Found status code means that the server either did not find a current representation for the requested resource or is trying to hide its existence from an unauthorized client. When looking at things SEO-wise the 404 Not Found status code pages with a high volume of traffic should be redirected using a 301 to the most relevant page possible. For some pages, however, a 404 might be necessary, for example, if the product is out of stock for an extended period of time. If you have external links pointing to a page that returns 404, you will lose the link equity those links would otherwise give.