Prepare the oranges: Remove the peel from 2 oranges with a vegetable peeler. Cut the white pith off the back with a very sharp knife. Cut into julienne, and blanch three times in boiling water. Drain and rinse under cold water. Set aside. Squeeze the juice of those 2 oranges and reserve. Peel and cut the remaining 2 oranges into sections, draining their juice into the other juice, and set the sections aside. You now have one dish of blanched julienned orange zest, about 1 cup/250 ml orange juice, and a dish of orange sections.
Prepare the duck breasts: Score the fat side of the breasts with a knife. Set them fat-side down in a pan over low heat and render the fat, about 10 minutes. Remove the duck and pour off the fat. Increase the heat in the pan and put the breasts back in skin-side down. Saute until done to your liking, or about 7 minutes on the fat side, then another 3 on the other. Remove to a carving board to rest, covering to keep warm.
Make the sauce: Put the sugar and 1 tablespoon water in a saucepan, bring to a boil and cook until golden, about 3 minutes. Add the vinegar and orange juice. Reduce slightly. Now add the stock and the zests. Boil down to sauce consistency. Remove from the heat and whisk in the butter. Check the seasonings. Add the orange sections. Carve the duck breasts and arrange on a serving platter. Spoon over the sauce, and serve.