Rewrite Custom Post Type URL slug

If the city and region is always city and region strings, you can do something like this.

For your specific case, you could do it like this:

'rewrite'            => array( 'slug' => 'city/region/event' ),
'hierarchical'       => true,
'has_archive'        => false,
'supports' => array( 'title', 'editor', 'thumbnail', 'page-attributes'),

If the city becomes some city and the region becomes some region in the URL i.e. city becomes London and region becomes South East, you have to use global $wp_rewrite and write the regular expression to change add the slugs.

You can find more documentation on WordPress Codex – https://codex.wordpress.org/Class_Reference/WP_Rewrite.