What query to append the 10 next posts?

Question

You need the $paged param and pass it with the ajax call.

$queryLoadPosts = new WP_Query(array(
  'post_type' => 'track',
  'posts_per_page' => 4,
  'paged' => (isset($_REQUEST['paged']) && intval($_REQUEST['paged'])) ? $_REQUEST['paged'] : 1
));

First time you click the button the paged param must be 2, second time 3, and so on… so you need to store somewhere the last value of paged with js, use a data- attribute of the button can be a good idea.

see Pagination Parameters of WP_Query on Codex