Featured image in admin on custom post type

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);