generate an auto incremented id number when a new user is registered

Question

You can use the same method as described in the link you mentioned, but simply use the created user ID and add 1000 so you get four digits. So user_id = 5 gets the meta number of 1005.

add_action( 'user_register', 'my_on_user_register' );
function my_on_user_register( $user_id ) {
    $unique_id = 1000 + $user_id;
    update_user_meta( $user_id, 'my_unique_id', $unique_id );
}

Hope that helps!