Rub the hens with 2 tablespoons of the olive oil and season them with salt and pepper. In a large skillet, heat the remaining 2 tablespoons of olive oil. Add the hens 1 at a time and brown them over moderately high heat, about 4 minutes per side. Transfer the hens to a large roasting pan and let cool slightly. Pour off the fat from the skillet. Add 1/2 cup of the wine to the skillet and boil over high heat for 2 minutes, using a wooden spatula to scrape up the brown bits from the bottom.
Preheat the oven to 400°. Using a citrus zester, zest 3 of the lemons. Alternatively, remove the zest from 3 of the lemons using a vegetable peeler, then cut the zest into fine julienne. Thinly slice the remaining 2 lemons.
Rub the hens with the rosemary. Put 2 garlic-clove halves and one-third of the lemon slices in each of the hen cavities. Tie the legs together and fold the wing tips underneath. Scatter the lemon zest over the hens. Pour the reduced wine into the roasting pan and add the remaining garlic cloves. Roast the hens for about 1 hour, basting after 15 minutes with 1/4 cup of wine. Baste 2 more times, until an instant-read thermometer inserted in an inner thigh registers 160°. Transfer the guinea hens to a carving board, cover loosely with foil and let rest for 10 minutes.
Pour the pan juices into a glass measuring cup and skim off the fat. Set the roasting pan over 2 burners. Add the remaining 1/2 cup of wine and boil over high heat, scraping up the brown bits, until almost evaporated. Pour in the reserved pan juices and simmer the sauce for 2 minutes. Strain the sauce into a warmed gravy boat and season with salt and pepper. Carve the hens and serve with the pan sauce.
This elegant dish requires a full-bodied, mature wine. A Brunello di Montalcino will have the necessary size.