Having Both NOBLOGREDIRECT and 404 Handling

I am running a WP Multisite 3.9.2 using sub-domains and it works great! Here is my problem, though. I have registrations disabled (private sites, no subscribers wanted). When someone goes to a true “404”, I want the “normal” 404 page to show. When someone types an invalid or previously used sub-domain, I would like it to go there, too. By default, it goes to wp-login.php and says registrations are disabled – true, but totally visitor unfriendly.

I tried setting NOBLOGREDIRECT to a page that says yeah, you goofed, that sub-domain does not exist, but then, also, all other 404’s go to that same page. Am I missing something? I would have no problem with it if there was some way in that page to differentiate between the two errors (internal variable, refer, etc.), but after reading through the codex, I cannot find a way.

Any assistance is appreciated!

Jim