That is our spring break, but I think the biggest spring break crowd will be for the couple of weeks after that. You can only book three in advance, but then you have to book one at a time. At MK, there will be some decent options for the 4th and maybe even a 5th or 6th. As a PP said, the kicker is that if you have a specific ride you want and get the 4th for a whole lot later, then you aren't going to have a good chance for a 5th. It definitely is not like the old days and in park FP+s are a whole lot harder to get. Our strategy at MK is typically to book the 3 we really want for that day as early as we can get them, then we look for the 4th and unless there is a particular one that is really important that day, we look at the times and get the earliest one for a ride with a longer line (which since FP+ can be a lot of them in our experience). We ride that and then try for something else. Sometimes you get lucky and get a FP+ that kicks in soon, sometimes for anything useful it is hours later. It is annoying because where we used to do areas at a time, we now find ourselves having to criss-cross the park to avoid standing in longer ride lines. You not only have to criss-cross to get to whatever ride you were lucky enough to get a FP+ for, but you also have to somewhat go out of your way to get to the kiosks. We are not big fans of FP+. I would say book your 3 for the most important rides with the longest lines just assuming you may not even get a 4th.
I will also warn you that while you can get a decent 4th and even 5th at MK, that we have never once been able to get anything as a 4th FP+ at DHS even stacking our FP+ early, and even though we do not go at high crowd times. EPCOT, about the only things we have ever been able to get a 4th FP+ for are Figment and Spaceship Earth- but while those weren't useful in the past and in the times we went we always walked on Figment and Spaceship Earth- now we have seen lines of 20 or 30 minutes for Figment more than once. So, while they seem like throw away FP+, they actually have come in handy.