Logging in issues

Copying content over from the old iPad to the Mini 4 means that you are restoring to a backup (unless you are only manually copying each app, song etc. onto it). Did your existing id used to be without the 's' (i.e. have you changed/updated it), or did you have a second account ? If you try that id (without the 's') on http://appleid.apple.com / https://iforgot.apple.com/password/verify/appleid#!&section=appleid does it find it ?

How is the email address 'wrong', and how did you set up the iPad, did you restore to a backup ? Being asked for an account's password (assuming that it's a different account) implies that you have content from that account on the iPad.

Is it showing an email that you recognize? Is the email you are referring to just randomly popping up in a notification? If so, chances are it is an AppleID that is linked to content on your device that was brought back through a backup restore. Meaning at some point in the past that content was downloaded on that device with that AppleID. The only way to get rid of it is two options: one you delete a single content (app, music, video) one at a time until the notification goes away. The second option is to erase your phone completely and set up as new. If you set up as new, and sign into your AppleID, you will still get contacts, calendars, notes etc back, just not apps, music, videos, messages etc. But that would be the best option for ensuring the message doesn’t come back. Hope this helps!

A couple of basic questions have been asked that might fix the problem. Was this your Apple ID at one time? Are there purchases on the new iPad that may have been downloaded using that Apple ID?