Yes you can upgrade. While I was waiting to get a card ticket from my online voucher, a person in front of me was doing a complicated upgrade with many tickets.
However, I would not get 4 days unless you are not doing full days. I have a three day and would have been fine with 2. So check how your days go before upgrading.
There is a lot for under 10 kids, not so much thrill rides, so if you have younger kids maybe 4 days will be warranted.