A quick reminder. There are two types of 404 errors:
A standard 404: When a page that does not exist is requested from a website the server responds with a page-not-found error code that is called a 404 Error. It is not anything bad; it is simply normal behavior that occurs when a page request cannot be fulfilled because the requested page does not exist.
A Soft 404: A soft 404 happens when a non-existent page is accessed and either displays a page not found message or redirects to an alternate page (like Home) but does not signal a proper 404-server error code.
From the horses mouth: English Google Webmaster Central office-hours hangout (Sep 9 2016)
Some sites have pages where they don't always have content to display. Google marks these as soft 404's. What are the consequences?
Ideally these pages should respond with a 404 error and they should use a No Index tag but if the do return a 200 response and Google then calls the page a soft 404 then: 'A soft-404 is OK... this is not the end of the world. It is just taken by Google as a signal not to index the page.' John Mueller.
Watch at about 2:00 mins into this vid: https://www.youtube.com/watch?v=Wcnz1kCoiks