PHILADELPHIA — Though the designation would probably make him uncomfortable, Adam Granduciel is — on paper, at least — what passes these days for a rock star.

As the lead singer and songwriter for the War on Drugs, one of the least likely breakout acts of this decade, Mr. Granduciel has taken an old-fashioned concept — the rangy, six-string-centric American band, steeped in reverence and grandiosity — and made it newly relevant, with more guitar solos than narrative or musical gimmicks.

On the strength of the War on Drugs’ sweeping 2014 album, “Lost in the Dream,” Mr. Granduciel, an unassuming frontman with a tangle of shoulder-length curls, went from toiling on the local indie circuit to headlining international music festivals and even signing a major-label record deal, a rarity for a 38-year-old traditionalist heading into his fourth LP.

“Maybe there was a void for a split second and then we were there,” Mr. Granduciel said last month, declining to muse much on the state of modern rock ’n’ roll, which has all but disappeared from the Billboard charts. “I don’t think about it too much,” he insisted.