Verse 4 repeats the calling discussed in verse 3. These priests were called according to their faith. At this point, the emphasis is not on the premortal preparation, but the mortal teaching. During mortality, they, and all humankind, are free to choose between good and evil. The priests were called according to their faith, but others would harden their hearts.
In verse 5, “in the first place they were on the same standing with their brethren” refers to the state of humanity under agency. All are equally able to choose. The ability to receive both the understanding, and then the blessing of the Atonement, depends upon choosing good. Those who harden their hearts do not have access to the redemptive aspect of the Atonement.
Alma links these foreordained priests to the mission of the Messiah. The Messiah was prepared from the foundation of the earth. Those who were to preach of his coming were also prepared from the foundation of the earth. This is not a new message; it is one of the oldest.
Returning to the ordination of the priests, Alma declares that they were ordained specifically to “teach his commandments.” While many of the children of Israel, and presumably those of Ammonihah, believed that those were Jehovah’s commandments to Moses, Alma declares that it was really to prepare for the commandments of Christ.