The elevation of veteran Swiss composer Jürg Frey, a key figure of the post-Cagean Wandelweiser Collective, came after Montréal’s Quatuor Bozzini premiered Frey’s second string quartet back in 2001. In the decades since, they’ve continued to work together — most recently on last year’s sublime Continuité, fragilité, résonance, where they formed a gauzy, heavenly alliance with the reed ensemble Konus Quartett. The arrival of Frey’s fourth string quartet is grounds for celebration. The intensely intimate, gently voiced epic doesn’t necessarily unveil new procedures or ideas — the molasses-slow pace and harmonic grandeur of the early works felt radical at the time — but it does feel like a stunning refinement and a generous expansion of those sounds. Most of the five-movement work is hushed, the strings producing a faint glow of sound that radiates a gauzy rainbow of color. Naturally, in music designed to be relatively static and still, any change or shift carries weight. When the quartet raises the intensity, as in the opening passages of the third movement, the sound hits hard, like a sudden explosion cracking open new possibilities—including an arrestingly beautiful solo violin line drifting through the sorrowful tumble of stately chords. But when nothing is happening, that emptiness is exquisite, carving out a space for contemplation that creates the most ravishing place on earth during its placid unfolding.