In the fourth aliyah, just like the previous aliyah, G-d tells Moshe and Aharon to separate themselves from the community so G-d can destroy them in an instant. And again Moshe and Aharon fall on their faces. But this time Moshe tells Aharon to quickly take a firepan and incense and go into the midst of the congregation because Moshe says that a plague is beginning while they speak. Aharon runs to do this and stops the plague. Actually, Rashi says that once when Moshe visited heaven, an angel told him this technique of using incense to stop a plague. Rashi also says that when Aharon went into the midst of the people he grabbed and physically restrained the angel making the plague. The angel complained and asked that he be permitted to do his job. Aharon told him that Moshe commanded him to stop the angel. The angel said that he was sent by G-d and Aharon was only sent by Moshe. Aharon replied that Moshe only speaks the will of G-d and he pointed out that G-d and Moshe are standing together, and that the angel can go over to them and ask about it.