function mwe_calculate_discount( $total_discount, $cart ) {

    //global $woocommerce;
    $num_products_required_for_discount = 4;

    echo "<!-- Discount? -->";

    $number_products = $cart->cart_contents_count;
    if ($number_products >= $num_products_required_for_discount) {

        echo "<!-- QUALIFIES FOR DISCOUNT! -->";

        for ($i = 0; $i < $number_products; $i++) {

            // Get the reduced price of the product - TODO

            // Calculate $product_discount
            $product_discount = 11.11; // TODO

            // Add $product_discount to the $total_discount
            $total_discount += $product_discount;
        }

        echo "<!-- Total discount: $total_discount -->";

    }
    else {

        echo "<!-- NO DISCOUNT -->";

    }

    return $total_discount;

}
add_filter('woocommerce_cart_total_discount', 'mwe_calculate_discount', 10, 2);