As far as it is known, there are more extant sources for Martinů’s Fourth Symphony than any of his other symphonic works. Examining these sources reveals a series of changes Martinů made in his score since its first completion in June 1945. Even though the changes are not very substantial, they were sufficiently significant to alter an interpreter’s view of the orchestration of the work; in particular, the role of the piano within the orchestral texture. Collecting and examining the sources per se have also led to a better understanding of performance, editorial and publishing practices in Martinů’s time, and consequently, a clearer view of the composer’s intentions behind the sheer notations in the score.