ShootingStar83 on "child-of [x] category in CURRENT post"

I found this code to show sub categories of [x] category and I like it, I just need to make it working as "Show sub-categories of [x] category IN CURRENT POST" because I'm going to paste this code in a widget inside posts only.

<?php
$subcategories = get_categories('&child_of=4&hide_empty'); // List subcategories of category '4' (even the ones with no posts in them)
echo '<ul>';
foreach ($subcategories as $subcategory) {
  echo sprintf('<li><a href="%s">%s</a></li>', get_category_link($subcategory->term_id), apply_filters('get_term', $subcategory->name));
}
echo '</ul>';
?>

What is the code that will apply this code to the post the visitor is reading?

Appreciate your help in advance.

Thank you