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If the 404 link has an equivalent on site, I'd update the link to point at the new equivalent. Even if the page isn't ranking, there's potential for a visitor to get to the page, so why not send them to proper content if you have it.

There's also a Moz Blog post regarding 404 pages: https://moz.com/blog/are-404-pages-always-bad-for-seo 

We have a client with multiple locations, and Google Business listings for each location. We've also created AMP pages for each of those location landing pages, and they're ranking on page one of their main queries, just below the local pack.

We're wondering if anyone has tried pointing a GMB website listing at an AMP page, or if there are considerations to keep in mind before testing it out. So far, we've discussed whether to use the AMP url (amp.domain.com) or the Google viewer url (google.com/amp/s/amp.domain.com), and potential concerns for desktop users.

If the 404 link has an equivalent on site, I'd update the link to point at the new equivalent. Even if the page isn't ranking, there's potential for a visitor to get to the page, so why not send them to proper content if you have it.

There's also a Moz Blog post regarding 404 pages: https://moz.com/blog/are-404-pages-always-bad-for-seo