Tom Johnson is really a minimalist composer; in fact, he coined the term while serving as the new music critic for the Village Voice.

He works with simple forms, limited scales, and generally reduced materials, but he proceeds in a more logical way than most minimalists, often using formulas, permutations, predictable sequences and various mathematical models.

The Four Note Opera (1972) is a work written using four notes only (D, E, A, B). It is scored for 5 singers and a piano (no orchestra) and the singers play the role of singers in a way similar to Pirandello’s Six Characters In Search of an Author:

The only sure thing is that the crucial moment in the evolution of the piece was that evening very long ago when I read, with great excitement, Luigi Pirandello’s Six Characters In Search of an Author. Normally characters are not even conscious of their existence on a stage. They are completely obedient to the author, they conform totally to the world the author creates, and they have no thoughts of their own. But Pirandello’s masterpiece was different. His characters knew they existed in a theatrical space, and only for a couple of hours. They were aware of the audience, and of the author as well. It was not the kind of theatre that asks you to believe something that is not true. It was the kind of theatre that you have to believe, because everything is true.

Pirandello’s vision had a strong impact on me, and for years one question lingered in the back of my mind: what would happen if, instead of Six Characters in Search of an Author, there happened to be some opera characters looking for a composer? It happened that some opera characters were looking for a composer, and about 10 years after reading Pirandello they found me and came to life in The Four Note Opera.
[Tom Johnson]

Here are some excerpts in french and italian:

and the whole