Trump voters in the flyover states are often referred to as “the forgotten” — the men and women whom the media and other presidential candidates failed to understand. An entire cottage industry has materialized around them, intending to “humanize” them, or explain their perspective to a baffled coastal audience. J. D. Vance, author of Hillbilly Elegy, is typically seen as the prototype of this group. But Will Arbery’s riveting off-Broadway play, Heroes of the Fourth Turning, dramatizes an even more obscure group, a forgotten sub-category within “the forgotten” — a group Rod Dreher, whose influence weighs heavily on the play,...