Graham Fitkin, an otherwise prolific composer of chamber operas and orchestral and large ensemble works, has written comparatively few pieces for string quartet. All six to date are featured here. The opening 1992 work, “Servant,” is thrilling, rhythmic, driven, its textures gradually opening out into a glorious, compelling explosion of dazzling harmony and polyphony. “Inside” is a multifaceted, restless accomplishment that constantly surprises and delights, while “A Small Quartet” achieves both introspection and mischievousness. The final piece, “Pawn,” concludes this superbly detailed, committed recording on an uneasy but poignant note.