A 5th park comes with a ton of additional problems for Disney.
- They have to invest even more on infrastructure to take and bring guests from their hotel rooms to the new park. Something Disney isn't great in investing in.
- A new park is a whole new operation with it's own restaurant, cleaning, landscaping, etc staff.
- The new park with all the new rides will be another competing factor to their current parks meaning adding rides there to bring people back.
It would be smarter to invest in new rides that put more butts in seat in their current parks so at least 2500+ guests/hour.
Invest new rides in unused locations and bring life to empty buildings. All the parks combined have worth a collection of empty buildings that is coming close to a new theme park. They are doing it now (Paris Rat, Space restaurant, Ellen's Energy, Galaxy Edge) but there is a ton to do and add before they need to think of a new theme park.