Hello Susan! Welcome to planDisney! Thank you for bringing such a great question to us! I'm happy to help!
I needed to get some clarification regarding your really good question. I reached out to a friendly Cast Member at Disney World. Unfortunately, using your 4-Day, 4-Park Magic Ticket to visit Animal Kingdom earlier in the day, and then purchasing an individual ticket to visit EPCOT later, won't be possible. If you already have a 4-Day, 4-Park ticket, the system will recognize that you already have an active ticket, and you wouldn't be able to add an individual ticket.
You are correct that
Animal Kingdom tends to close earlier than the other parks. However, while I'm not sure which date you're visiting, there are some evenings, particularly in the summer, when Animal Kingdom stays open later for Guests to enjoy!
If you check the Disney Parks calendar (linked here), you can check the date of your visit and the park operating hours for your preferred date! With any luck, Animal Kingdom might be open a little bit later for you!
Susan, here is a PRO TIP for you! If you're looking to enjoy some "extended time" at Animal Kingdom, you might consider booking a later dining reservation. Even if the park closes, you can remain at your dining location. After your dinner reservation, you would then need to make your way out of the park. My absolute favorite dining location at Animal Kingdom is
Yak and Yeti Restaurant! The food is some of my favorite in all Disney World!
Dreams do come true!
Jonathan