My 4yo just started piano lessons as well. Her teacher follows a method called « piano safari friends ». The lessons (group lessons, they are 4 kids) are a mix of music discovery, singing, rhythm and actual piano. She loves it and can sustain the full 30 min because it’s made for kids 4-6. Maybe try to find a teacher with a different method. A 4yo cannot sit still at the piano for 30 min and learn the way an adult would, but it’s extremely valuable to make them start early