Related Posts: Changing Function For Posts Per Page

I have gone through your code and found that the function longform_the_related_posts displays 8 related posts. This function is defined in functions.php of parent theme and called in single.php.

Thus, to override the function, follow below steps –
1. Create another function that displays 4 related posts in functions.php of child theme. (As you have already created function longform_the_related_posts_change() )
2. Copy the file single.php from parent to child theme and replace the line 47


echo longform_the_related_posts();


echo longform_the_related_posts_change();

This would call the function that display 4 posts.
Hope this helps.