I imagine it would only work for public apps, it can’t work as an alternative for non-public ones because we don’t have a native way for now.
When a user signs in, alongside other types of info in their user profiles, have a column for their first name and a column for the “4 digits”. Use a template column to combine them.
Then create a “sign in” tab, with a text entry writing to a user-specific column telling them to input their username (the template column above) or sign in with email (use the sign in button).
If user is signed in or the text entry matches a row in your sheet then show them the right content.