Добавьте общую стоимость продукта в аккумуляторную переменную.
<?php
$total = 0;
foreach ($_SESSION as $products) {
echo "<tr>";
echo "<td class='product_name'> " . $products[0] . "</td>";
echo "<td class='product-price' > $" . $products[1] . "</td>";
echo '<td class="product_quantity">' . $products[2] . '</td>';
echo '<td class="product_total">$' . intval($products[3]) . '</td>';
$total += intval($products[3]);
echo "</tr>";
}
echo "<td>Total</td><td colspan='2'></td><td>$" . $total . "</td>";
?>