John Mueller said on twitter that if you are using 301 redirect then prior to this be confirmed that you are using it for 1:1 relationship between URL otherwise it will be considered as 404 page.

What is 1:1 relationship?

For instance you have a page for a product ABC and you need to change the URLs then redirect it to new page for the same product ABC then it is called 1:1 replacement or relationship.

But if you the new pages is concerned to some other product ABD then it is not a 1:1 relationship.

Non 1:1 replacement will be treated as 404 page

If Google finds any page which after the changing of URL is not fulfilling the criteria of 1:1 then it will treat it as 404 pages.

301-redirecting for 404s makes sense if you have 1:1 replacement URLs, otherwise we’ll probably see it as soft-404s and treat like a 404.

— John ☆.o(≧▽≦)o.☆ (@JohnMu) 25 June 2017