I don't believe it's possible to use WP_Query
to filter based on the post type of the parent. The most straightforward way around this is to use SQL and $wpdb->get_results()
directly. This will also bypass all built in caching, etc so think about how your implementation will be used.
global $wpdb;
// the SQL statement to only fetch the N last attachments
$sql = <<<SQL
SELECT attachments.*
FROM {$wpdb->posts} attachments
-- join the parent post based on the parent_post value to filter by cpt
JOIN {$wpdb->posts} post
ON post.ID = attachments.post_parent
AND post.post_status = 'publish'
-- pass in the custom post type
AND post.post_type = %s
WHERE
-- only fetch images, double percents for prepare()
attachments.post_mime_type LIKE 'image%%'
ORDER BY attachments.post_date DESC
-- pass in the number to fetch
LIMIT %d
SQL;
$cpt = "your_custom_post_type";
$limit = 4;
$attachments = $wpdb->get_results( $wpdb->prepare( $sql, $cpt, $limit ) );
foreach ( $attachments as $attachment ){
// $attachment will have all the rows from the posts table.
$id = $attachment->ID;
}