orderby comment_count being ignored

I’m trying to loop through the 4 most commented posts with the following code:-

<?php	$mcQuery = new WP_Query ( array ('orderby' => 'comment_count', 'order' => 'DESC', 'posts_per_page' => '4' ) );
		if ( $mcQuery->have_posts ()) :
			while ( $mcQuery->have_posts () ) :
				$mcQuery->the_post (); ?>

Stuff……..

<?php			endwhile;
                 endif;
		 wp_reset_postdata (); ?>

All I get back is the 4 most recent posts, posts_per_page working as expected but not the orderby.

Anyone have any suggestions as to what is happening or spot anything I am doing wrong?