“The Last Encores” at NJPAC presents a live rock/pop set rooted in the Frankie Valli & The Four Seasons songbook, with Valli and the group’s legacy at the center of the night. It follows a lineage tied to New York and New Jersey’s postwar club circuit, where this sound first consolidated into a recognizable pop vernacular. The show functions less as revival than as documentation in real time, testing how well harmony-driven hits hold up outside their original era. One thing becomes clear: the material endures through structure and vocal arrangement, not nostalgia alone.