Here are a few lines of code from a wordpress plugin (WP E-commerce) that calculate the price. Now what i want is that if 'cart_item' are more than 3 in quantity then the price should be (number of items * 4) else the function should work as it is listed below. Thnks for help.

function calculate_subtotal($for_shipping = false) {
1047      global $wpdb;
1048      if($for_shipping == true ) {
1049              $total = 0;
1050              foreach($this->cart_items as $key => $cart_item) {
1051                if($cart_item->uses_shipping == 1) {
1052                      $total += $cart_item->total_price;
1053                  }
1054              }
1055      } else {
1056          $total = 0;
1057          if($this->subtotal == null) {
1058              foreach($this->cart_items as $key => $cart_item) {
1059                  $total += $cart_item->total_price;
1060              }
1061              $this->subtotal = $total;
1062          } else {
1063              $total = $this->subtotal;
1064          }
1065      }
1066          return $total;
1067    }