If you have only 4 level, and each third level term has children, the we can determine the fourth level term if have children:
$term_id = get_queried_object()->term_id;
$taxonomy_name="product_range";
$custom_terms = get_term_children( $term_id, $taxonomy_name );
foreach ( $custom_terms as $custom_term ) {
$term = get_term_by( 'id', $custom_term, $taxonomy_name );
$terms_chilren = get_term_children( $term_id, $taxonomy_name );
wp_reset_query();
// if we can not get a term`s children, we can confirm the term is the fourth level term.
if ( ! empty( $terms_chilren ) && ! is_wp_error( $terms_chilren ) ) {
$args = [
'post_type' => 'product',
'posts_per_page' => 1,
'tax_query' => [
[
'taxonomy' => 'product_range',
'field' => 'slug',
'terms' => $term->slug,
'parent' => 0,
],
],
];
}
}