Single User Login without Multisite?

Hello!

I have a main site plus four subdomains, and am looking into using http://codecanyon.net/item/userpro-user-profiles-with-social-login/5958681 to handle memberships for various reasons.

My question is this: I want users to be able to use the same login for all four subdomains. Is this possible without converting to a multi-site network? That really isn’t an option for multiple reasons (huge databases, etc.).

Would it be possible to have a custom plugin developed that bridges them, so when a user registers on sub1.eg.com, their info is also registered at sub2.eg.com, sub3.eg.com, and sub4.eg.com? I’d need it to work so whenever someone registers on any of the subdomains (sub1, sub2, sub3, or sub 4) that they’re also registered on the other three.

Is this possible?
Thanks in advance.