READERS' PICK: You know that VW commercial where every sidewalk denizen and storekeeper is in step with a syncopated rhythm that only the passengers hear and see? Fourth Avenue's sidewalks pulse a similar visual soundtrack, but without the percussive windshield wipers keeping time. Watch and listen: Street kids curb-sit with their pit bull pups; suburban families sequestered in Caruso's garden await Sunday night dinner; crunchy types pour out of the co-op with overflowing canvas bags; a whole panoply of folks spill out of Value Village. We're fascinated with the newest arrival on the Avenue--the storefront music producer and purveyor of baggie pants, out of which emanates audible urban rhythms. Thump ... thump, thump go the pulsating speakers. Lowriders cruise by, blaring what's left of music distorted by too much bass. It's a veritable symphony out here--a synesthesia for the eyes and ears.