: While not exclusively about Four! , this article by Don Byron offers a technical deep dive into Henderson's unique improvisational style, explaining why his playing sounds "episodic yet cohesive". Album Context

: A primary critical source that describes the album as a "frequently exciting performance." It highlights how Henderson "really pushes the rhythm section"—the legendary Wynton Kelly Trio—and how he was inspired by their decade-long chemistry.

: April 21, 1968, live at the Left Bank Jazz Society in Baltimore.