Second in a series of three records of Library Music miniatures from composer and multi-instrumentalist Daniel O'Sullivan (Æthenor, Ulver, This Is Not This Heat, etc) following 2020's Electric Māyā. For heads, the term "Library Music" in 2021 might evoke dodgy Italian gray market LPs and crate-diggers hunting for "funky breaks"-but London's venerable KPM Music is working with groundbreakers like O'Sullivan to open up new avenues for composers to experiment with. Fourth Density's fifteen tracks include several beguiling instrumental beauties, including the Ashra-like "Astral Survivor" and the drifty "Faster Than Light." Mixed in with the instrumental are almost-pop gems like the hypnotic "Orgone Attenuation" and "Head In The Belfry," both with guest vocals from Astrud Steeholder. Like the other volumes, this is in a beautiful jacket designed by O'Sullivan and Turner Prize-nominee Mark Titchner and pressed on aesthetically complimentary blue vinyl.