Craft CMS 3 admin panel returns 404 with SSL
tl;dr: In my case, adding AllowOverride All back in to httpd.conf fixed the issue.

I was having the same issue on my staging server. I noticed that, in addition to /admin throwing a 404, all of my site's subpages were 404-ing. This cause me to try the unrouted index.php url for a subpage (mysite.com/index.php?p=subpage) and it worked! Note, this wasn't working for /index.php?p=admin because it was properly routing to /admin/login. Plugging in /index.php?p=admin/login worked. 

I ran through the Craft guide for removing index.php from urls. Everything was set up properly, but my server admin found that AllowOverride All had been commented out of httpd.conf during a server update and was never un-commented. Adding AllowOverride All back in to httpd.conf fixed the issue.