How to modify Related Posts function

Fantastic theme.
Following up on previous post about removing Related Posts, I would like to modify the Related Posts function as follows:

For all posts in category ID 4, I want to show in the Related Posts section posts from category ID 5 (yes, they are not related posts by tag or category in the strictest sense). My assumption is that I have to include an if/else section inside the if ( ot_get_option(‘related-posts’) == ‘categories’ ) part of functions.php

Can anybody provide the revised code? THANKS!

/*  Related posts
/* ------------------------------------ */
if ( ! function_exists( 'alx_related_posts' ) ) {

	function alx_related_posts() {
		wp_reset_postdata();
		global $post;

		// Define shared post arguments
		$args = array(
			'no_found_rows'				=> true,
			'update_post_meta_cache'	=> false,
			'update_post_term_cache'	=> false,
			'ignore_sticky_posts'		=> 1,
			'orderby'					=> 'rand',
			'post__not_in'				=> array($post->ID),
			'posts_per_page'			=> 3
		);
		// Related by categories
		if ( ot_get_option('related-posts') == 'categories' ) {

			$cats = get_post_meta($post->ID, 'related-cat', true);

			if ( !$cats ) {
				$cats = wp_get_post_categories($post->ID, array('fields'=>'ids'));
				$args['category__in'] = $cats;
			} else {
				$args['cat'] = $cats;
			}
		}
		// Related by tags
		if ( ot_get_option('related-posts') == 'tags' ) {

			$tags = get_post_meta($post->ID, 'related-tag', true);

			if ( !$tags ) {
				$tags = wp_get_post_tags($post->ID, array('fields'=>'ids'));
				$args['tag__in'] = $tags;
			} else {
				$args['tag_slug__in'] = explode(',', $tags);
			}
			if ( !$tags ) { $break = true; }
		}

		$query = !isset($break)?new WP_Query($args):new WP_Query;
		return $query;
	}

}