How to get notified when your pages are broken?

Sometimes I notice (randomly) that some pages on a site I'm working on are broken/ have become 404s.

I always fix the issue and bring them back to normal, but by the time I notice the problem, hours have gone by.

Is there a way (maybe GA) to get notified every time something like "x users have visited 404s in the past y minutes" happens?

Edit: Just to be clear, I need something that notifies me while it's happening (rather than running an audit that tells me which pages are 404s... I know those tools but that's not what I'm looking for).