The four sons of Mosiah were not the only members of this noble missionary band. Earlier, we learned that these four men took some trusted friends along with them (Mosiah 28:1). The total number is undisclosed but Muloki and Ammah are some of the friends of the sons of Mosiah. Again, the names of the sons of Mosiah are Ammon, Aaron, Omner, and Himni.
One might find it odd, that these three are found in prison together when they all separated at the beginning of their missions (Alma 17:17). However, the next chapter explains that Aaron started out alone and met up with Muloki, Ammah, and others during his travels. Just as they were beginning to have some success in the land of Middoni, the three mentioned (Aaron, Muloki, and Ammah) were cast into prison.