get_post_ancestors() returns all of the ancestors hierarchically. As you only want to display the 2nd level post title on 4th post page, you can do something like –
global $post;
$parents = get_post_ancestors( get_the_ID() );
if( count($parents) == 4 ){
// ancestor1 = 1st parent, ancestor2 = second.
echo get_the_title($parents['1']); // index starts from 0, 1 = 2nd item
}