Swiss tubaist Marc Unternahrer provided the inspiration for the exchange noted in this foursome’s moniker, but the group’s album, Several Lights (Delmark), is ultimately a landmark for Chicago music: it’s a fantastic album of free improvisation, one of the most powerful statements yet by the wave of post-Ken Vandermark players who gravitated here in the late 90s. Unternahrer, a dazzling player whose extended technique on his unwieldy ax conveys a low, trombonely range and a great deal of blubbery-sound imagination, spent five months here in 2002 as part of the Chicago Sister Cities International Program, quickly hooking up with a large crew of local players. He played most often with cornetist Josh Berman, tenor saxophonist Keefe Jackson, and drummer Frank Rosaly, and returned here last summer to record Several Lights with them. Considering that the quartet had no tunes and no preset melodic or harmonic structures, it’s astonishing how compositional the tracks feel; the players clearly have a deep rapport, but it takes an especially refined sensitivity for so many serendipitous collisions to sound so harmonious. And unlike many free-improv players in Chicago, this group isn’t ashamed to draw upon its collective love of jazz. Rosaly plays with a feather-stroke touch, dropping a hint of pulse here or a splash of color there, while Jackson’s marbled tone expresses a lovely lyricism no matter how brusque or serrated his lines get. Berman and Unternahrer play quicksilver, high-low brass games: the cornetist excels on the instrument’s lower registers and scratches up his liquid high notes, while the tubaist simulates snatches of bass lines and blows floor-rumbling long tones that resonate with the snaking lines that Jackson and Berman produce. Unternahrer was unable to make the trip here for the group’s record-release show, but the remaining trio has enough ideas to chew on by themselves. Sun 4/24, 10 PM, Hungry Brain, 2319 W. Belmont, 773-935-2118, donation requested.