Woody Guthrie, Pete Seeger, Leadbelly, and for the fourth......hmm... Bill Monroe? Jimmie Rodgers? Jean Ritchie? Dylan? Could be any of those, or...?. Not a "best" list, but a "very significant" list of those who brought/brings this music in some of its American forms to the fore. I'm assuming that this thread is asking for U.S. musicians for the "Rushmore" type sculpture? Hard enough to name four, let alone making it an international sculpture. I treat this as Guest Paul does - it's just a hypothetical question.