Query posts with thumbnail or any attachments within a certain category

gistfile1.php

<?php
global $wpdb;
$possible_ids = $wpdb->get_col($wpdb->prepare("SELECT `post_parent` FROM {$wpdb->$posts} WHERE `post_parent`!=0 AND `post_type`='attachment' GROUP BY `post_parent`;"));


$args = array(
    'post__in' => $possible_ids,
    'posts_per_page' => 4,
    'category__in' => array(8),
    'meta_query' => array(
        array(
            'key' => '_thumbnail_id'
        ),
    )
);
$q = WP_Query($args);