Thank you for this! The teaser about Branca's 4 led me to look for any other recordings. While I've not found solid evidence of it, this 1983 show in Zurich is the right era (he was touring the fifth symphony the next year), and the fourth piece is "unknown": archive.org/details/Bran...

Glenn Branca Live - 1983-05-14 Houdini, Zürich, Switzerland : Glenn Branca : Free Download, Borrow, and Streaming : Internet Archive

A very good recording of a piece by modern classical innovator Glenn Branca - lots of droning and ambient harmonics from his loud guitar band as they play...

That "unknown" fourth track certainly seems like it'd fit as one of the movements of the microtonal 4th. And it's just beautiful.