Bring some sunscreen and your best dance moves. This Saturday, from 1 to 8 p.m. at the South Street Seaport, the fourth annual 4Knots Music Festival will offer free live music and dancing for all ages. Organized by The Village Voice in collaboration with the Howard Hughes Corporation, this year’s event will showcase more than 10 renowned and upcoming musicians, including Dinosaur Jr., Mac Demarco, Dead Stars and Speedy Ortiz. Performances will take place on two stages, one on Pier 16 and the other near Pier 17. The show will happen, rain or shine. The name of the festival refers to the speed at which the East River flows.