I've been very successful with using MFPA for 4 year olds provided they can recognize letters A through G, numbers 1-5 and identify colors. Progress is much slower as Jon said. It could take up to a year to complete book A alone and up to 3 years to complete the entire MFPA series. But that's ok--it's fun and gives them plenty of practice! By the time they get to level one, they have a very solid foundation.
With 4 year olds, I constantly have them review previous songs, especially when practicing at home with parents. We also interact with piano friends and use highlighter tape for quicker note recognition.

I do not recommend MFPA for 3 year olds as the progression is too fast. 3 year olds need to spend a lot of time with LH/RH games, identifying piano keys and letters and finger numbers. Bastien has a preschool method (Piano Party) and there is also Pianimals. Hope that helps!
_________________________
"You are musical if you have music not in your fingers only, but in your head and heart." ~Schumann