Hi Maria!
That is a GREAT question that has a lot of variables, however, I am going to give you my best advice! If you are willing to move at a brisk pace throughout the parks, not sleep very much, take advantage of
Extra Magic Hours, and use
FastPass+, it is completely possible to do a park each day. My husband and I can spend a day at Disney's Hollywood Studios and a day at Disney's Animal Kingdom and feel like we didn't miss anything. Although this may be a little harder with the addition of
Pandora - The World of Avatar at Disney's Animal Kingdom and the other lands coming soon to Disney's Hollywood Studios. Fitting everything in all in one day may be a little trickier with Epcot and Magic Kingdom Park since these are typically the parks in which my husband and I like to spend a little more time.
All of that being said, my best advice is to buy
tickets for 5 days and add the Park Hopper Option. This gives you the time to go to at least one park each day, the flexibility to switch parks if you choose, and an extra day at the end of your vacation to go back to see anything you may have missed earlier in your trip! If you would like to also go to Disney's Blizzard Beach Water Park or Disney's Typhoon Lagoon Water Park, I would suggest planning to spend a day at each of those! If water parks sound like something you would like to add to your trip, then make sure to look into the Park Hopper Plus Option!
Have a Great Big Beautiful Tomorrow!
-Lindsey N.