In a 10- to 12-inch frying pan over high heat, stir garlic in 2 teaspoons oil until sizzling, 1 to 2 minutes. Add spinach (in batches), and broth. Cover, and cook until tender; keep warm.
Meanwhile, rinse bass, pat dry, and cut into 4 equal pieces. Mix kosher salt and pepper together and rub mixture on fish. Pour remaining 2 teaspoons oil into a 10- to 12-inch frying pan over high heat. When oil is hot, in about 1 minute, lay bass in pan; cook 3 minutes. With a wide spatula, turn fish and cook until it is opaque but still moist-looking in center of thickest part (cut to test), about 3 minutes more.
Place bass in wide bowls and spoon bok choy and broth equally around fish.