RobertMueller

Genius!
Passing the posts_per_page argument does the trick!
You saved may day! Thanks!

– Set posts_per_page to -1 for displaying all posts without limit,
– Removed ‘limit’ argument passed to get_tptn_pop_posts, since I don’t want any limit in number of posts and setting it to FALSE changed the ordering.

By the way: Including a category filter in the standard shortcode and widget would be another killer feature of Top 10 in my opinion …

Here’s the final code for anyone trying to reproduce thist solution:

function tptn_shortcode_ablagesystem( $atts, $content = null ) {
	if ( function_exists( 'get_tptn_pop_posts' ) ) {

		$list ='';

		$settings = array(
			'daily' => TRUE,
			'daily_range' => 30,
			'strict_limit' => FALSE,
		);

		$topposts = get_tptn_pop_posts( $settings ); // Array of posts

		$topposts = wp_list_pluck( $topposts, 'postnumber' );

		$args = array(
			'post__in' => $topposts,
			'orderby' => 'post__in',
			'post_type' => 'post',
			'cat' => '4',
			'posts_per_page' => '-1',
		);

		$my_query = new WP_Query( $args );
		if ( $my_query->have_posts() ) {
			$list = '<ul>';
			while ( $my_query->have_posts() ) {
				$my_query->the_post();
				$list = $list . '<li><a href="' . get_permalink( get_the_ID() ) . '">' . get_the_title() . '</a></li>';
				wp_reset_postdata();
			}
			$list = $list . '</ul>';

		} else {
		}
		wp_reset_query();
		return $list;
	}
}
add_shortcode( 'tptn_list_ablagesystem', 'tptn_shortcode_ablagesystem' );