Midrasch zu Wajikra 9:15

9) (Vayikra 9:8): "And Aaron drew near to the altar": with zeal! "and he slaughtered the calf of the sin-offering which was his": He started with his offering. (Vayikra 9:15): "And he presented the offering of the people": When he finished with his offerings, he came to the offering of the people. Know that his offering atones more than does the offering of the people. For the offering of the people atones only for themselves, whereas his offering atones both for himself and for the people, viz.: (Vayikra 9:7): "And offer your sin-offering and your burnt-offering and make atonement for yourself and for the people; and offer the offering of the people and make atonement for them." (Vayikra 9:15): "And he slaughtered it (the sin-offering of the people) and he offered it as a sin-offering, like the first (his calf)": Just as the fist required chitui (application of the blood on the horn with the finger), so this required chitui. Just as the first required four applications, this, too, required four applications.