A custom WordPress nav walker class for Bootstrap 4 (v4.0.0-alpha.2) nav menus in a custom theme using the WordPress built in menu manager
// Register Custom Navigation Walker
require_once('bs4navwalker.php'); <?php
// Use the new walker
wp_nav_menu([
'menu' => 'primary',
'theme_location' => 'primary',
'container' => 'div',
'container_id' => 'exCollapsingNavbar2',
'container_class' => 'collapse navbar-toggleable-sm',
'menu_id' => false,
'menu_class' => 'nav navbar-nav',
'depth' => 2,
'fallback_cb' => 'bs4navwalker::fallback',
'walker' => new bs4navwalker()
]);