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<?php cat_is_ancestor_of( $cat1, $cat2 ); ?> 

<?php 
  // if the category is music or a music SUBcategory, 
  if (cat_is_ancestor_of(4, $cat) or is_category(4)):  ?>
  <div id="music_subnav_menu" class="subnav_menu">
    <?php wp_nav_menu( array('menu' => 'Music' )); ?>
  </div>
<?php endif; ?>