I tried both codes and they worked for me,
first I added this code
add_filter('manage_posts_columns', 'bootpress_slider_columns');
function bootpress_slider_columns($defaults) {
$defaults['featured_image'] = 'Featured Image';
return $defaults;
}
and it added column both in custom post type and on posts
Then I followed this link manage_${post_type}_posts_columns
my post type is “slider”
the below code worked for me
add_filter('manage_slider_posts_columns', 'bootpress_slider_columns');
function bootpress_slider_columns($defaults) {
$defaults['featured_image'] = 'Featured Image';
return $defaults;
}
in your case the below code should work
add_filter('manage_events_posts_columns', 'ST4_columns_content', 10, 2);