For Drupal 7: Domain access would make more sense and you could share content between sites as well as views, themes, styles, modules.
In 8 Group (not organic groups) uses a domain access feature as well (a separate module).
You mentioned multisite in the title which is another option (share code, separate dbs).
This is difficult, because it is core functionality.
What you could do to prevent people from checking node/4, node/5 etc is blocking access when a url request has a certain structure. You'd need a custom module with an EventSubscriber.
Here is an example how to intercept such url requests and how to redirect them. In this case it is a taxonomy term view but ...