Pour the beans into a deep bowl and pour in enough cold water to cover them by 4 inches. Let soak in a cool place or the refrigerator at least 8 hours or overnight.

Drain the beans and transfer them to a 2-quart saucepan. Pour in enough water to cover by two fingers and drop in two of the rosemary sprigs. Bring the water to a boil, then lower the heat so the water is at a bare simmer. Cook until the beans are tender but not mushy, with just enough liquid to cover them, 30 to 40 minutes. (If necessary, add more water a tablespoon at a time to keep the beans covered as they simmer.)

Remove the beans from the heat and gently stir in the oil, salt to taste, and the remaining 2 rosemary sprigs. Let stand to cool and absorb the cooking liquid. The end result should be tender beans with a creamy consistency in just enough liquid to coat them. Taste the beans occasionally as they cool and stir in more salt if necessary.