In a Large, heavy pot, stir the popcorn kernels and 3 tablespoons of the rosemary-infused oil. Cover and cook over medium-high heat until the kernels have popped, shaking the pot halfway through cooking, about 3 minutes. Immediately transfer the popcorn to a large bowl. Toss the popcorn with the remaining tablespoon of oil. Sprinkle with salt to taste, and serve.