Hi there! You sure can add a fourth day. You can upgrade your 3-Day Park Hopper ticket to any other ticket of equal or greater value. You need to purchase the upgrade before the completion of your third visit. For 2017 Southern California CityPASS, a credit of $315 for ages 10+ and $303 for ages 3-9 will be applied toward the other ticket. The credit will be the same for 2018 CityPASS when it goes on sale in the next couple of weeks. You can purchase the upgrade at
Disneyland. Let me know if you have any other questions. Hope that helps!
Alicia