Before formal piano lessons, music educators widely recommend rhythm and ear-training classes to build a musical foundation. These group classes are specifically designed for ages 3-5.
This is the honest answer most piano teachers won't tell you upfront: a 4-year-old often benefits more from Kodály or Eurhythmics classes than sitting at a piano bench. Build the ear first, then the fingers.
“At this age, I suggest Kodaly or Eurythmics classes instead of actual piano lessons. It is not that kids can't learn to play, but the progress”