Hi, Christina!
To visit more than one park per day, you must add the Park Hopper option onto your
theme park ticket. If you try to enter a second park (even if using a second ticket) the system will deny your entry. You are allowed to upgrade your ticket, but the option will cost about $75 (plus tax, subject to change). If you decide to
upgrade your ticket with the Park Hopper option, you can stop by any ticket window, Guest Relations, or your resort lobby concierge and a cast member can assist you.
Since you will have an extra day, try to use it if possible. Even if you pop into a park for an hour or so on your arrival day or do an early morning "rope drop" before heading home, use that ticket. Every bit of magic counts when you are Walt Disney World! Some of my family's fondest memories at Disney are the spur of the moment "let's run into a park before it closes" moments! It is just bonus time to add to the fun.
Wishing you 4 (or 5!) magical days in
Walt Disney World!
Cathy