Google not monitor content on the 404 pages
Google scans the content of a page when it sees that the server returns a 404 error. This was stated by the representative of the search John Mueller during the last hangout for webmasters. So to fill these pages with content from the point of view of search engine optimization does not make much sense.
"So if we see a 404, then we see a 404 and don’t look at the content. We don’t look to see what is visible on your 404 pages, on your server error pages, we essentially assume that something that the user can look at and kind of deal with, we don’t follow the content on pages that return any of these error response codes." — explained Mueller
Custom 404 page useful. They help to keep visitors by redirecting them to other pages of the website. However, some webmasters use 404 page for search engine optimization.
According to the Google guide, assessors who evaluate the quality of search results users need to see the contents of the 404 page and how it is useful. Therefore, some webmasters used the 404 page to improve the quality of the resource. Now, however, it became clear that this parameter is not so important for search engines, as previously thought.
Again, from the point of view of user experience quality 404 page is important since it helps to keep visitors on the website and help them to find useful information. But what is on these pages, Google is not looking.
Listen to Muller's response, starting at 58:34 video