Dynamically insert code to custom post type loop

Create a variable outside the array and then pass it to the array:

<?php 
$catfilm = get_post_meta( get_the_ID(), "mytheme_catfilm", true );
$blog = array(
    'post_type'         => 'film',
    'catfilm'           => $catfilm,                       
    'post_status'       => 'publish',
    'posts_per_page'    => 4,
    'ignore_sticky_posts' => 1,
    );
?>