That's because you are using v4.0.x, that code is meant for v4.1.x
In /include/product_modify.php look for this instead...
Code:
#
# Update product data
#
db_query("UPDATE $sql_tbl[products] SET product='$product', descr='$descr', fulldescr='$fulldescr', avail='$avail', list_price='$list_price', weight='$weight', productcode='$productcode', forsale='$forsale', distribution='$distribution', free_shipping='$free_shipping', shipping_freight='$shipping_freight', discount_avail='$discount_avail', min_amount='$min_amount', return_time = '$return_time', low_avail_limit='$low_avail_limit', free_tax='$free_tax' WHERE productid='$productid'");
Add the new fields like this...
Code:
newfield='$newfield',
HTH