Dealing with Product options in eCart 4 will be the same as in eCart 5.
eCart 5 offers a couple more features like the checkout wizard creating the orders and order details tables for you if you choose, but if eCart 4 is working without a problem, it is not an imperative upgrade. it is up to your discretion.
if you choose to upgrade, you will still be able to manage PHP carts created in eCart 4 using eCart. If you have carts in other languages though, ecart 5 will not work with them.