A ‘fourth panel’ is a way to transcend an art form. Traditionally, a painting is a square piece of canvas with colored paint. But if you are very clever, like Titus Kaphar and Ken Gonzales in their painting “Beyond the Myth of Benevolence,” you can add a physical as well as an historical dimension. This is their painting in the National Art Gallery which shows Thomas Jefferson on a curtain and behind him, Sally Hemmings, the black slave who was Jefferson’s ‘mistress.’