Yes, your express pass is good from day you check in through end of park time, the day you check out. As we saw, the last holiday weekend was busy. What this Easter week will look like, not sure, as who knows if park attendance is greater, and if there will be later park times. If you have 4 day tickets, why not enjoy 4 park days? If it is busy, even with express, the lines can get long. But if you find the extra night too high cost, then the 3 and 1/2 days should be a good amount of time to enjoy most of the rides and shows at least once, and go back to enjoy those you want to go to or on again. If it is warm, pool time, relaxing should be part of those days. And on one of the days, take advantage of the early entry to the Studios for hotel guests. Even on busy days, will make the Potter area in the Studios not as crowded, and minions and maybe another ride, are open too. Your express does not work during that early hour, but lines are generally very quick during that time. Plus, you will be in the park to get in line to another ride opening when they open the gates. For the first hour after official opening, lines are reasonable, and express should be close to waking on. As the morning passes, lines increase, and by the lunchtime, crowds increase even more. With early park closing times, City Walk can then get busy too.
So, for me, I’d do the 3 and 1/2 days, abd then you can decide, if enough fun was had, turn your tickets into annual passes.
