Is it possible to get either of these as a 4th FastPass? We are going in April the week after Easter and I can't decide which one we can't live without...lol.
Your chances of getting a “day of” FP for Peter Pan is slim to none. Plan toward getting one for Thunder Mountain.
I would rope drop Peter Pan first as it is more central. Take the walkway up between Liberty Square and the Castle. Then go to Big Thunder. Being on the outer edge of MK, it would take longer for the queue to fill up.Better chance for Big Thunder but even that is iffy. Rope drop your first choice and then run to the other one. Use your FPs later in the day when it gets more crowded.
Do you have kids? You can get 4+ fast passes in advance if you have kids and take advantage of rider swap. If you have kids under the height requirement for big thunder mountain then one member of your party can get a fast pass for that ride and you can go to the line and they will give you a fast pass for the rest of your party without using any of your 3 fast passes. Then another member of your party could get splash mountain or mine train or whatever else you want in advance and do the same thing. Basically you could get 6 fast passes for your group if you have a small child and 2 adults. It wouldn't work for peter pan though unless there is a height restriction i'm unaware of.