Google doesnt index pages that return 404. If when crawling it found 404, it wouldnt index it.
... so if Google has a page indexed, and it now returns 404, it means Gogoel crawled and indexed it BEFORE it came 404.
So, you can't get a list of pages that has become 404 from Google since it indexed. Because well it doesnt KNOW about the 404 yet. As soon as it does it wont be indexed!
So would have to get a list of indexed URLs. And cross reference it with some OTHER list of URLs that are now 404.
You can't directly get a list of ALL indexed URLs, but if have < 1000 can possible get them from the Coverage Report. Ie it will list upto 1000 in each status.
... if have over 1000 urls indexed, then only get a sample from the console. Although sitemaps can be used to filter the coverage report. (but if you have a list in sitemaps, probably dont need the list from Google!)