I just recently did 4 days last year for the first time and it was perfect. We used to do 3 days, which felt fine, too, but 4 allowed us to get in just that much more, especially since the park was getting more crowded as the weekend came along. You can definitely get everything done in 2-3 days, though, at both parks. Three days is like the sweet spot. And if your body can handle one more day, then go for it. Since you're staying practically in the parks, it'll be a lot easier for you to pop into the hotel, take a nap, and recharge so I wouldn't worry too much about staying the full open to close, which is what I do since I can't afford staying on property and live out of state. We cram as much in as possible. It's just a matter of what your budget is and whether you'd like to do things like eat at nicer restaurants or buy more souvenirs. If money isn't a problem, though, you'd be fine with 4 days.