It's very busy at Christmas. Still, with proper use of FP, getting there at opening time, staying late, etc, over the course of 4 days you should be able to see/ride pretty much everything. Since it is a school break time, they will be open until at least 11 if not midnight. The only caveat I can think of is that that is the early part of the rainy season. Last Christmas day was some of the worst rain I've ever seen. Rain here often last all day or even for days at a time. I can't say that it will be raining during that time, but be aware it's a possibility. Of course, if it's raining, it'll keep the locals at home and, in DL anyway, a lot of the rides are indoors. If it has been raining for a couple of days and then it dawns sunny the next day, they'll be there in big numbers.