Pegging Chatham County Line as a “progressive” bluegrass combo is both accurate and misleading. On IV, the North Carolina foursome continues to adhere to traditional instrumentation and bask in an old-timey atmosphere, but lead singer/guitarist Dave Wilson’s bittersweet croon of a voice owes as much to Paul McCartney as to Bill Monroe or Ricky Skaggs, while his group’s songwriting stretches beyond typical bluegrass limitations. Closely matched harmonies and vigorous picking infuse “I Got Worry” and “Let It Rock” with a back-porch party spirit. The lilting “Chip of a Star” and the feisty “Whipping Boy” fuse British pop with Appalachian motifs. There are echoes of the Grateful Dead’s laid-back fatalism in “Sweet Eviction” and the Everly Brothers’ tender romanticism in “One More Minute.” “Birmingham Jail” audaciously — and effectively — revisits the ‘60s Civil Rights struggle in mountaineer ballad form. As producer, veteran cult rocker Chris Stamey encourages the band’s expansive urges, nudging them further into alt-pop realms. Call them blue-, new- or post-grass, Chatham County Line makes IV into a transcendent experience.