With this i have 4th level working, hope it will help you, to tweak it edit a $parent = $ancestors[0]; etc , also you can uncomment //echo to see whats going on and where :) regards
<?php if(is_page()){
$ancestors = get_post_ancestors($post);
if (count($ancestors) == 0) {
$children = wp_list_pages("sort_column=post_date&title_li=&child_of=".$post->ID."&echo=0&depth=1");
//echo 'test0';
}elseif (count($ancestors) == 1) {
$parent = $ancestors[0];
$children = wp_list_pages("sort_column=menu_order&title_li=&child_of=" .$parent. "&echo=0");
//echo 'test1';
}elseif (count($ancestors) == 2) {
$parent = $ancestors[1];
$children = wp_list_pages("sort_column=menu_order&title_li=&child_of=" .$parent. "&echo=0");
//echo 'test2';
}elseif (count($ancestors) == 3) {
$parent = $ancestors[2];
$children = wp_list_pages("sort_column=menu_order&title_li=&child_of=" . $parent. "&echo=0");
//echo 'test3';
}elseif (count($ancestors) == 4) {
$parent = $ancestors[3];
$children = wp_list_pages("sort_column=menu_order&title_li=&child_of=" . $parent . "&echo=0");
//echo 'test4';
}
?>
<?php if($children){ ?>
<ul class="leftmenu">
<?php echo $children; ?>
</ul>
<?php } }?>