In a small bowl, combine the red wine vinegar, water, sugar, and salt. Add the onion and let stand for 1 hour.
Unroll the slices of pancetta and wrap 2 pieces of pancetta around each radicchio quarter. Set aside.
Heat a 10- to 12-inch nonstick, sauté pan over medium heat for 1 minute. Gently place the radicchio in the pan and cook, turning frequently with tongs, until the pancetta is browned and crisp, 6 to 8 minutes.
Transfer to a plate. Sprinkle with salt and pepper, cover with foil, and keep in a warm place.
Drain any fat from the pan, and add the balsamic vinegar and olive oil. Bring to a boil and reduce by half. Remove from the heat.
Drain the onions, discarding the liquid, and arrange one-quarter of the onions in a small pile in the center of each plate. Lean the warm radicchio pieces up against the onions like a teepee, and spoon about 1 teaspoon balsamic sauce over each piece. Sprinkle with rosemary and serve immediately.