We were there for a week and didn't have time to do "everything", but it was also split with some side trips (Busch, Seaworld).
With the Express Passes, you can easily do all the rides in one park in one day and still have time left over, even with the early closing times. In the off season, if you just want to ride everything once and leave I think you could very easily do that in 2 days. However, we are very geeky tourists. We had to try every type of Butterbeer, try all the wand stations, look in every shop, and ride front, back, left and right on every coaster
To me that is the luxury of Uni with Express Passes, you have time to relax and do all the crazy things you really want to do. If you are just looking to ride once and leave, 4 days is probably way too much. 3 days seemed to be ideal for me during my first trip as I spent one day in each park and then the third day going back to redo the highlights.
During our December trip, we missed a lot because we had to fight the weather. We had one beautiful day, and then 2 days of non-stop rain. All we wanted to do at night was get back to a warm hotel room. Because of this we missed mini-golf in CityWalk, Cinema Spectacular, and the Hot Dog Hall of Fame. We were easily able to do all the "must-do" but missed a lot of the 2nd tier stuff we wanted to do.
P.S. While there was no issue in the parks with the early closing times, it did impact CityWalk as all the restaurants were slammed with the 7pm park closing time. The new Toothsome Emporium is a must do for me on my next trip so I'll probably pad an extra day for it and the new Sports Grill as I could easily spend a day watching sports, drinking beer, and eating chocolate