I think rejected is the wrong term because Ibn Masud's, radiya Allahu anhu, great status is definately acknowledged. Well, I am not going to speak on TripolySunni's behalf but one of the reasons I think is because they "are the companions who died much later" as per his words. I think a possible reason for not including Ibn Masud is because his status is more elevated from the other Abdullahs. Another reason is probably because when the people said, "That is the saying of the 'Abadillah", they simply meant the four; because they died much later than Ibn Masud, radia Allahu anhum.