A documentary focusing on a prodigiously gifted 4-year-old painter whose work has been called into question by journalists who believe her father helped her create her art is screening at Sundance, and it delves deeply (and disturbingly) into issues of both artistic and journalistic ethics. “The documentary gradually became a meditation on truth instead, one that manages to explore and sometimes indict the motives of all the adults who have swirled around Marla.”