In my personal experience Frozen FP always felt like a waste, and Muppets never needed one. BUT - if it was me and I had my heart set on a ride, that is where I would make the FP. You have some time to see how things change after SWGE opens, but I imagine Muppets will see an increase in guests because it is near to the entrance to the area. Will it need a FP? Sereiously doubtful. But if it was my 'must do' I would just go ahead and book a FP for it because I get anxious like that lol If Frozen and Muppets are both equally must do's then I would keep the Frozen one because like others have said Frozen does not repeat constantly.
Are your FPs all for morning/afternoon so you can try to pick up whatever isn't in your first three as a 4th? I think finding Muppets as a 4th wouldn't be an issue - at all.