Cover potatoes with salted cold water by 2 inches in a 5- to 6-quart pot and simmer, uncovered, until just tender, 20 to 25 minutes. Drain in a colander and cool slightly.
While potatoes are simmering, whisk together sugar and 3 tablespoons vinegar in a large bowl until sugar is dissolved.
When potatoes are just cool enough to handle, peel and cut diagonally into 1/2-inch-thick slices, adding to vinegar mixture as sliced and tossing gently to combine.
Whisk together shallot, mustard, and remaining tablespoon vinegar in a small bowl, then add oil in a slow stream, whisking until emulsified. Add dressing to potatoes, then season with salt and pepper and stir gently with a rubber spatula.
