NewtonUK, I think that's not altogether correct. For decades, the minimum for nonmusicals was four. If a play had more than a certain amount of live music, the union might decide that it should be classified as a musical and might demand that the minimum for musicals be hired, but that happened very rarely. An example would be The Tempest, way back in 1945. Later, if the rule was still in place, I imagine Marat/Sade would have been classified as a musical according to the union. But otherwise, even in the biggest houses (and the Broadway, the Uris/Gershwin, and the Majestic have all sometimes housed plays), the minimum was four for nonmusicals.