e-Commerce v4 and beyond requires PHP 5.2+ to run. We settled on PHP 5.2 be be inline with with http://phpgo5.org which is working on the initiative to move people away from PHP 4 to 5. See http://gophp5.org/faq#n7 which shows why they settled on PHP5.2
However you are able to run the core of e-Commerce v4 with the PHP_Compat package from http://pear.php.org. See http://pear.php.net/package/PHP_Compat for the package and installation instructions.
To set this up with your installation add the following code in your settings.php file for your site.
<?php
require_once 'PHP/Compat.php';
PHP_Compat::loadFunction(array('array_intersect_key', 'array_diff_key'));
?>