missing quote at the end of the id=4 file name and no quotes around the default file name.
PS: You could probably do that more cleanly with a switch or an array instead of a bunch of if/eleif/else statements.
$pages = array(
1 => 'about.php',
2 => 'plans.php',
3 => 'apply.php',
4 => 'support.php'
);
$file = (isset($pages[$id])) ? $pages[$id] : 'default.php';
include $file;