In a medium bowl, whisk together 1 tablespoon of the oil, miso, sugar and 1 tablespoon warm water to make a glaze. Brush catfish all over with some of the glaze. Heat ½ tablespoon oil in a large skillet over medium heat. Arrange two fillets in the skillet and cook, brushing with more glaze and pan juices and flipping once, until cooked through, about 5 minutes; transfer to a plate and keep warm. Wipe skillet clean then repeat process with 1/2 tablespoon more oil and remaining fillets. Reduce heat to medium; add bok choy to skillet and brush all over with remaining glaze. Add 1/4 cup water to skillet, cover and cook, tossing occasionally, until just wilted and bright green, 3 to 4 minutes. Arrange bok choy alongside catfish and serve.