The connection between Swiss composer Jürg Frey and the Montréal string ensemble Quatuor Bozzini keeps strengthening, as each party gains additional intimacy, introspection, and intuition for the other. There’s always the danger that such familiarity could breed laziness, and while the fourth string quartet by the composer might not feel as radical or daring as its predecessors, the rigor, beauty, and thoughtfulness have not become victims of the strengthening relationship. Bozzini have a way of articulating the composer’s most fragile passages with unerring sensitivity and lyric fragility, imbuing Frey’s darkly luminescent harmonies and sorrowful progressions with a lightness of touch that doesn’t preclude a sumptuous viscosity and grain, sound morphing with seamless ease, colors swirling and swelling.